12.4: How Diseases Spread - Biology

12.4: How Diseases Spread - Biology

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Learning Objectives

  • Describe the different types of disease reservoirs
  • Compare contact, vector, and vehicle modes of transmission
  • Identify important disease vectors
  • Compare different types of infectious diseases, including iatrogenic, nosocomial, and zoonotic diseases
  • Explain the prevalence of nosocomial infections

Understanding how infectious pathogens spread is critical to preventing infectious disease. Many pathogens require a living host to survive, while others may be able to persist in a dormant state outside of a living host. But having infected one host, all pathogens must also have a mechanism of transfer from one host to another or they will die when their host dies. Pathogens often have elaborate adaptations to exploit host biology, behavior, and ecology to live in and move between hosts. Hosts have evolved defenses against pathogens, but because their rates of evolution are typically slower than their pathogens (because their generation times are longer), hosts are usually at an evolutionary disadvantage. This section will explore where pathogens survive—both inside and outside hosts—and some of the many ways they move from one host to another.

Reservoirs and Carriers

For pathogens to persist over long periods of time they require reservoirs where they normally reside. Reservoirs can be living organisms or nonliving sites. Nonliving reservoirs can include soil and water in the environment. These may naturally harbor the organism because it may grow in that environment. These environments may also become contaminated with pathogens in human feces, pathogens shed by intermediate hosts, or pathogens contained in the remains of intermediate hosts.

Pathogens may have mechanisms of dormancy or resilience that allow them to survive (but typically not to reproduce) for varying periods of time in nonliving environments. For example, Clostridium tetani survives in the soil and in the presence of oxygen as a resistant endospore. Although many viruses are soon destroyed once in contact with air, water, or other non-physiological conditions, certain types are capable of persisting outside of a living cell for varying amounts of time. For example, a study that looked at the ability of influenza viruses to infect a cell culture after varying amounts of time on a banknote showed survival times from 48 hours to 17 days, depending on how they were deposited on the banknote.1 On the other hand, cold-causing rhinoviruses are somewhat fragile, typically surviving less than a day outside of physiological fluids.

A human acting as a reservoir of a pathogen may or may not be capable of transmitting the pathogen, depending on the stage of infection and the pathogen. To help prevent the spread of disease among school children, the CDC has developed guidelines based on the risk of transmission during the course of the disease. For example, children with chickenpox are considered contagious for five days from the start of the rash, whereas children with most gastrointestinal illnesses should be kept home for 24 hours after the symptoms disappear.

An individual capable of transmitting a pathogen without displaying symptoms is referred to as a carrier. A passive carrier is contaminated with the pathogen and can mechanically transmit it to another host; however, a passive carrier is not infected. For example, a health-care professional who fails to wash his hands after seeing a patient harboring an infectious agent could become a passive carrier, transmitting the pathogen to another patient who becomes infected.

By contrast, an active carrier is an infected individual who can transmit the disease to others. An active carrier may or may not exhibit signs or symptoms of infection. For example, active carriers may transmit the disease during the incubation period (before they show signs and symptoms) or the period of convalescence (after symptoms have subsided). Active carriers who do not present signs or symptoms of disease despite infection are called asymptomatic carriers. Pathogens such as hepatitis B virus, herpes simplex virus, and HIV are frequently transmitted by asymptomatic carriers. Mary Mallon, better known as Typhoid Mary, is a famous historical example of an asymptomatic carrier. An Irish immigrant, Mallon worked as a cook for households in and around New York City between 1900 and 1915. In each household, the residents developed typhoid fever (caused by Salmonella typhi) a few weeks after Mallon started working. Later investigations determined that Mallon was responsible for at least 122 cases of typhoid fever, five of which were fatal.2 See Eye on Ethics: Typhoid Mary for more about the Mallon case.

A pathogen may have more than one living reservoir. In zoonotic diseases, animals act as reservoirs of human disease and transmit the infectious agent to humans through direct or indirect contact. In some cases, the disease also affects the animal, but in other cases the animal is asymptomatic.

In parasitic infections, the parasite’s preferred host is called the definitive host. In parasites with complex life cycles, the definitive host is the host in which the parasite reaches sexual maturity. Some parasites may also infect one or more intermediate hosts in which the parasite goes through several immature life cycle stages or reproduces asexually.

George Soper, the sanitary engineer who traced the typhoid outbreak to Mary Mallon, gives an account of his investigation, an example of descriptive epidemiology, in “The Curious Career of Typhoid Mary.”

Exercise (PageIndex{1})

  1. List some nonliving reservoirs for pathogens.
  2. Explain the difference between a passive carrier and an active carrier.


Regardless of the reservoir, transmission must occur for an infection to spread. First, transmission from the reservoir to the individual must occur. Then, the individual must transmit the infectious agent to other susceptible individuals, either directly or indirectly. Pathogenic microorganisms employ diverse transmission mechanisms.

Contact Transmission

Contact transmission includes direct contact or indirect contact. Person-to-person transmission is a form of direct contact transmission. Here the agent is transmitted by physical contact between two individuals (Figure (PageIndex{1})) through actions such as touching, kissing, sexual intercourse, or droplet sprays. Direct contact can be categorized as vertical, horizontal, or droplet transmission. Vertical direct contact transmission occurs when pathogens are transmitted from mother to child during pregnancy, birth, or breastfeeding. Other kinds of direct contact transmission are called horizontal direct contact transmission. Often, contact between mucous membranes is required for entry of the pathogen into the new host, although skin-to-skin contact can lead to mucous membrane contact if the new host subsequently touches a mucous membrane. Contact transmission may also be site-specific; for example, some diseases can be transmitted by sexual contact but not by other forms of contact.

When an individual coughs or sneezes, small droplets of mucus that may contain pathogens are ejected. This leads to direct droplet transmission, which refers to droplet transmission of a pathogen to a new host over distances of one meter or less. A wide variety of diseases are transmitted by droplets, including influenza and many forms of pneumonia. Transmission over distances greater than one meter is called airborne transmission.

Indirect contact transmission involves inanimate objects called fomites that become contaminated by pathogens from an infected individual or reservoir (Figure (PageIndex{2})). For example, an individual with the common cold may sneeze, causing droplets to land on a fomite such as a tablecloth or carpet, or the individual may wipe her nose and then transfer mucus to a fomite such as a doorknob or towel. Transmission occurs indirectly when a new susceptible host later touches the fomite and transfers the contaminated material to a susceptible portal of entry. Fomites can also include objects used in clinical settings that are not properly sterilized, such as syringes, needles, catheters, and surgical equipment. Pathogens transmitted indirectly via such fomites are a major cause of healthcare-associated infections.

Clinical Focus: Resolution

Based on Michael’s reported symptoms of stiff neck and hemiparesis, the physician suspects that the infection may have spread to his nervous system. The physician decides to order a spinal tap to look for any bacteria that may have invaded the meninges and cerebrospinal fluid (CSF), which would normally be sterile. To perform the spinal tap, Michael’s lower back is swabbed with an iodine antiseptic and then covered with a sterile sheet. The needle is aseptically removed from the manufacturer’s sealed plastic packaging by the clinician’s gloved hands. The needle is inserted and a small volume of fluid is drawn into an attached sample tube. The tube is removed, capped and a prepared label with Michael’s data is affixed to it. This STAT (urgent or immediate analysis required) specimen is divided into three separate sterile tubes, each with 1 mL of CSF. These tubes are immediately taken to the hospital’s lab, where they are analyzed in the clinical chemistry, hematology, and microbiology departments. The preliminary results from all three departments indicate there is a cerebrospinal infection occurring, with the microbiology department reporting the presence of a gram-positive rod in Michael’s CSF.

These results confirm what his physician had suspected: Michael’s new symptoms are the result of meningitis, acute inflammation of the membranes that protect the brain and spinal cord. Because meningitis can be life threatening and because the first antibiotic therapy was not effective in preventing the spread of infection, Michael is prescribed an aggressive course of two antibiotics, ampicillin and gentamicin, to be delivered intravenously. Michael remains in the hospital for several days for supportive care and for observation. After a week, he is allowed to return home for bed rest and oral antibiotics. After 3 weeks of this treatment, he makes a full recovery.

Vehicle Transmission

The term vehicle transmission refers to the transmission of pathogens through vehicles such as water, food, and air. Water contamination through poor sanitation methods leads to waterborne transmission of disease. Waterborne disease remains a serious problem in many regions throughout the world. The World Health Organization (WHO) estimates that contaminated drinking water is responsible for more than 500,000 deaths each year.3 Similarly, food contaminated through poor handling or storage can lead to foodborne transmission of disease (Figure (PageIndex{3})).

Dust and fine particles known as aerosols, which can float in the air, can carry pathogens and facilitate the airborne transmission of disease. For example, dust particles are the dominant mode of transmission of hantavirus to humans. Hantavirus is found in mouse feces, urine, and saliva, but when these substances dry, they can disintegrate into fine particles that can become airborne when disturbed; inhalation of these particles can lead to a serious and sometimes fatal respiratory infection.

Although droplet transmission over short distances is considered contact transmission as discussed above, longer distance transmission of droplets through the air is considered vehicle transmission. Unlike larger particles that drop quickly out of the air column, fine mucus droplets produced by coughs or sneezes can remain suspended for long periods of time, traveling considerable distances. In certain conditions, droplets desiccate quickly to produce a droplet nucleus that is capable of transmitting pathogens; air temperature and humidity can have an impact on effectiveness of airborne transmission.

Tuberculosis is often transmitted via airborne transmission when the causative agent, Mycobacterium tuberculosis, is released in small particles with coughs. Because tuberculosis requires as few as 10 microbes to initiate a new infection, patients with tuberculosis must be treated in rooms equipped with special ventilation, and anyone entering the room should wear a mask.

Vector Transmission

Diseases can also be transmitted by a mechanical or biological vector, an animal (typically an arthropod) that carries the disease from one host to another. Mechanical transmission is facilitated by a mechanical vector, an animal that carries a pathogen from one host to another without being infected itself. For example, a fly may land on fecal matter and later transmit bacteria from the feces to food that it lands on; a human eating the food may then become infected by the bacteria, resulting in a case of diarrhea or dysentery (Figure (PageIndex{4})).

Biological transmission occurs when the pathogen reproduces within a biological vector that transmits the pathogen from one host to another (Figure (PageIndex{4})). Arthropods are the main vectors responsible for biological transmission (Figure (PageIndex{5})). Most arthropod vectors transmit the pathogen by biting the host, creating a wound that serves as a portal of entry. The pathogen may go through part of its reproductive cycle in the gut or salivary glands of the arthropod to facilitate its transmission through the bite. For example, hemipterans (called “kissing bugs” or “assassin bugs”) transmit Chagas disease to humans by defecating when they bite, after which the human scratches or rubs the infected feces into a mucous membrane or break in the skin.

Biological insect vectors include mosquitoes, which transmit malaria and other diseases, and lice, which transmit typhus. Other arthropod vectors can include arachnids, primarily ticks, which transmit Lyme disease and other diseases, and mites, which transmit scrub typhus and rickettsial pox. Biological transmission, because it involves survival and reproduction within a parasitized vector, complicates the biology of the pathogen and its transmission. There are also important non-arthropod vectors of disease, including mammals and birds. Various species of mammals can transmit rabies to humans, usually by means of a bite that transmits the rabies virus. Chickens and other domestic poultry can transmit avian influenza to humans through direct or indirect contact with avian influenza virus A shed in the birds’ saliva, mucous, and feces.

Exercise (PageIndex{2})

  1. Describe how diseases can be transmitted through the air.
  2. Explain the difference between a mechanical vector and a biological vector.

Using GMOs to Stop the Spread of Zika

In 2016, an epidemic of the Zika virus was linked to a high incidence of birth defects in South America and Central America. As winter turned to spring in the northern hemisphere, health officials correctly predicted the virus would spread to North America, coinciding with the breeding season of its major vector, the Aedes aegypti mosquito.

The range of the A. aegypti mosquito extends well into the southern United States (Figure (PageIndex{6})). Because these same mosquitoes serve as vectors for other problematic diseases (dengue fever, yellow fever, and others), various methods of mosquito control have been proposed as solutions. Chemical pesticides have been used effectively in the past, and are likely to be used again; but because chemical pesticides can have negative impacts on the environment, some scientists have proposed an alternative that involves genetically engineering A. aegypti so that it cannot reproduce. This method, however, has been the subject of some controversy.

One method that has worked in the past to control pests, with little apparent downside, has been sterile male introductions. This method controlled the screw-worm fly pest in the southwest United States and fruit fly pests of fruit crops. In this method, males of the target species are reared in the lab, sterilized with radiation, and released into the environment where they mate with wild females, who subsequently bear no live offspring. Repeated releases shrink the pest population.

A similar method, taking advantage of recombinant DNA technology,4 introduces a dominant lethal allele into male mosquitoes that is suppressed in the presence of tetracycline (an antibiotic) during laboratory rearing. The males are released into the environment and mate with female mosquitoes. Unlike the sterile male method, these matings produce offspring, but they die as larvae from the lethal gene in the absence of tetracycline in the environment. As of 2016, this method has yet to be implemented in the United States, but a UK company tested the method in Piracicaba, Brazil, and found an 82% reduction in wild A. aegypti larvae and a 91% reduction in dengue cases in the treated area.5 In August 2016, amid news of Zika infections in several Florida communities, the FDA gave the UK company permission to test this same mosquito control method in Key West, Florida, pending compliance with local and state regulations and a referendum in the affected communities.

The use of genetically modified organisms (GMOs) to control a disease vector has its advocates as well as its opponents. In theory, the system could be used to drive the A. aegypti mosquito extinct—a noble goal according to some, given the damage they do to human populations.6 But opponents of the idea are concerned that the gene could escape the species boundary of A. aegypti and cause problems in other species, leading to unforeseen ecological consequences. Opponents are also wary of the program because it is being administered by a for-profit corporation, creating the potential for conflicts of interest that would have to be tightly regulated; and it is not clear how any unintended consequences of the program could be reversed.

There are other epidemiological considerations as well. Aedes aegypti is apparently not the only vector for the Zika virus. Aedes albopictus, the Asian tiger mosquito, is also a vector for the Zika virus.7 A. albopictus is now widespread around the planet including much of the United States (Figure (PageIndex{6})). Many other mosquitoes have been found to harbor Zika virus, though their capacity to act as vectors is unknown.8 Genetically modified strains of A. aegypti will not control the other species of vectors. Finally, the Zika virus can apparently be transmitted sexually between human hosts, from mother to child, and possibly through blood transfusion. All of these factors must be considered in any approach to controlling the spread of the virus.

Clearly there are risks and unknowns involved in conducting an open-environment experiment of an as-yet poorly understood technology. But allowing the Zika virus to spread unchecked is also risky. Does the threat of a Zika epidemic justify the ecological risk of genetically engineering mosquitos? Are current methods of mosquito control sufficiently ineffective or harmful that we need to try untested alternatives? These are the questions being put to public health officials now.


Individuals suspected or known to have been exposed to certain contagious pathogens may be quarantined, or isolated to prevent transmission of the disease to others. Hospitals and other health-care facilities generally set up special wards to isolate patients with particularly hazardous diseases such as tuberculosis or Ebola (Figure (PageIndex{7})). Depending on the setting, these wards may be equipped with special air-handling methods, and personnel may implement special protocols to limit the risk of transmission, such as personal protective equipment or the use of chemical disinfectant sprays upon entry and exit of medical personnel.

The duration of the quarantine depends on factors such as the incubation period of the disease and the evidence suggestive of an infection. The patient may be released if signs and symptoms fail to materialize when expected or if preventive treatment can be administered in order to limit the risk of transmission. If the infection is confirmed, the patient may be compelled to remain in isolation until the disease is no longer considered contagious.

In the United States, public health authorities may only quarantine patients for certain diseases, such as cholera, diphtheria, infectious tuberculosis, and strains of influenza capable of causing a pandemic. Individuals entering the United States or moving between states may be quarantined by the CDC if they are suspected of having been exposed to one of these diseases. Although the CDC routinely monitors entry points to the United States for crew or passengers displaying illness, quarantine is rarely implemented.

Classifications of Disease

The World Health Organization’s (WHO) International Classification of Diseases (ICD) is used in clinical fields to classify diseases and monitor morbidity (the number of cases of a disease) and mortality (the number of deaths due to a disease). In this section, we will introduce terminology used by the ICD (and in health-care professions in general) to describe and categorize various types of disease.

An infectious disease is any disease caused by the direct effect of a pathogen. A pathogen may be cellular (bacteria, parasites, and fungi) or acellular (viruses, viroids, and prions). Some infectious diseases are also communicable, meaning they are capable of being spread from person to person through either direct or indirect mechanisms. Some infectious communicable diseases are also considered contagious diseases, meaning they are easily spread from person to person. Not all contagious diseases are equally so; the degree to which a disease is contagious usually depends on how the pathogen is transmitted. For example, measles is a highly contagious viral disease that can be transmitted when an infected person coughs or sneezes and an uninfected person breathes in droplets containing the virus. Gonorrhea is not as contagious as measles because transmission of the pathogen (Neisseria gonorrhoeae) requires close intimate contact (usually sexual) between an infected person and an uninfected person.

Diseases that are contracted as the result of a medical procedure are known as iatrogenic diseases. Iatrogenic diseases can occur after procedures involving wound treatments, catheterization, or surgery if the wound or surgical site becomes contaminated. For example, an individual treated for a skin wound might acquire necrotizing fasciitis (an aggressive, “flesh-eating” disease) if bandages or other dressings became contaminated by Clostridium perfringens or one of several other bacteria that can cause this condition.

Diseases acquired in hospital settings are known as nosocomial diseases. Several factors contribute to the prevalence and severity of nosocomial diseases. First, sick patients bring numerous pathogens into hospitals, and some of these pathogens can be transmitted easily via improperly sterilized medical equipment, bed sheets, call buttons, door handles, or by clinicians, nurses, or therapists who do not wash their hands before touching a patient. Second, many hospital patients have weakened immune systems, making them more susceptible to infections. Compounding this, the prevalence of antibiotics in hospital settings can select for drug-resistant bacteria that can cause very serious infections that are difficult to treat.

Certain infectious diseases are not transmitted between humans directly but can be transmitted from animals to humans. Such a disease is called zoonotic disease (or zoonosis). According to WHO, a zoonosis is a disease that occurs when a pathogen is transferred from a vertebrate animal to a human; however, sometimes the term is defined more broadly to include diseases transmitted by all animals (including invertebrates). For example, rabies is a viral zoonotic disease spread from animals to humans through bites and contact with infected saliva. Many other zoonotic diseases rely on insects or other arthropods for transmission. Examples include yellow fever (transmitted through the bite of mosquitoes infected with yellow fever virus) and Rocky Mountain spotted fever (transmitted through the bite of ticks infected with Rickettsia rickettsii).

In contrast to communicable infectious diseases, a noncommunicable infectious disease is not spread from one person to another. One example is tetanus, caused by Clostridium tetani, a bacterium that produces endospores that can survive in the soil for many years. This disease is typically only transmitted through contact with a skin wound; it cannot be passed from an infected person to another person. Similarly, Legionnaires disease is caused by Legionella pneumophila, a bacterium that lives within amoebae in moist locations like water-cooling towers. An individual may contract Legionnaires disease via contact with the contaminated water, but once infected, the individual cannot pass the pathogen to other individuals.

In addition to the wide variety of noncommunicable infectious diseases, noninfectious diseases (those not caused by pathogens) are an important cause of morbidity and mortality worldwide. Noninfectious diseases can be caused by a wide variety factors, including genetics, the environment, or immune system dysfunction, to name a few. For example, sickle cell anemia is an inherited disease caused by a genetic mutation that can be passed from parent to offspring (Figure (PageIndex{8})). Other types of noninfectious diseases are listed in Table (PageIndex{2}).

Table (PageIndex{2}): Types of Noninfectious Diseases
InheritedA genetic diseaseSickle cell anemia
CongenitalDisease that is present at or before birthDown syndrome
DegenerativeProgressive, irreversible loss of functionParkinson disease (affecting central nervous system)
Nutritional deficiencyImpaired body function due to lack of nutrientsScurvy (vitamin C deficiency)
EndocrineDisease involving malfunction of glands that release hormones to regulate body functionsHypothyroidism – thyroid does not produce enough thyroid hormone, which is important for metabolism
NeoplasticAbnormal growth (benign or malignant)Some forms of cancer
IdiopathicDisease for which the cause is unknownIdiopathic juxtafoveal retinal telangiectasia (dilated, twisted blood vessels in the retina of the eye)

Lists of common infectious diseases can be found at the following Centers for Disease Control and Prevention (CDC), World Health Organization (WHO), and International Classification of Diseases websites.

Exercise (PageIndex{3})

  1. Describe how a disease can be infectious but not contagious.
  2. Explain the difference between iatrogenic disease and nosocomial disease.

Healthcare-Associated (Nosocomial) Infections

Hospitals, retirement homes, and prisons attract the attention of epidemiologists because these settings are associated with increased incidence of certain diseases. Higher rates of transmission may be caused by characteristics of the environment itself, characteristics of the population, or both. Consequently, special efforts must be taken to limit the risks of infection in these settings.

Infections acquired in health-care facilities, including hospitals, are called nosocomial infections or healthcare-associated infections (HAI). HAIs are often connected with surgery or other invasive procedures that provide the pathogen with access to the portal of infection. For an infection to be classified as an HAI, the patient must have been admitted to the health-care facility for a reason other than the infection. In these settings, patients suffering from primary disease are often afflicted with compromised immunity and are more susceptible to secondary infection and opportunistic pathogens.

In 2011, more than 720,000 HAIs occurred in hospitals in the United States, according to the CDC. About 22% of these HAIs occurred at a surgical site, and cases of pneumonia accounted for another 22%; urinary tract infectionsaccounted for an additional 13%, and primary bloodstream infections 10%.9 Such HAIs often occur when pathogens are introduced to patients’ bodies through contaminated surgical or medical equipment, such as catheters and respiratory ventilators. Health-care facilities seek to limit nosocomial infections through training and hygiene protocols such as those described in previous chapters.

Exercise (PageIndex{4})

Give some reasons why HAIs occur.

Key Concepts and Summary

  • Reservoirs of human disease can include the human and animal populations, soil, water, and inanimate objects or materials.
  • Contact transmission can be direct or indirect through physical contact with either an infected host (direct) or contact with a fomite that an infected host has made contact with previously (indirect).
  • Vector transmission occurs when a living organism carries an infectious agent on its body (mechanical) or as an infection host itself (biological), to a new host.
  • Vehicle transmission occurs when a substance, such as soil, water, or air, carries an infectious agent to a new host.
  • Healthcare-associated infections (HAI), or nosocomial infections, are acquired in a clinical setting. Transmission is facilitated by medical interventions and the high concentration of susceptible, immunocompromised individuals in clinical settings.


  1. 1 Yves Thomas, Guido Vogel, Werner Wunderli, Patricia Suter, Mark Witschi, Daniel Koch, Caroline Tapparel, and Laurent Kaiser. “Survival of Influenza Virus on Banknotes.” Applied and Environmental Microbiology 74, no. 10 (2008): 3002–3007.
  2. 2 Filio Marineli, Gregory Tsoucalas, Marianna Karamanou, and George Androutsos. “Mary Mallon (1869–1938) and the History of Typhoid Fever.” Annals of Gastroenterology 26 (2013): 132–134.
  3. 3 World Health Organization. Fact sheet No. 391—Drinking Water. June 2005.
  4. 4 Blandine Massonnet-Bruneel, Nicole Corre-Catelin, Renaud Lacroix, Rosemary S. Lees, Kim Phuc Hoang, Derric Nimmo, Luke Alphey, and Paul Reiter. “Fitness of Transgenic Mosquito Aedes aegypti Males Carrying a Dominant Lethal Genetic System.” PLOS ONE 8, no. 5 (2013): e62711.
  5. 5 Richard Levine. “Cases of Dengue Drop 91 Percent Due to Genetically Modified Mosquitoes.” Entomology Today.
  6. 6 Olivia Judson. “A Bug’s Death.” The New York Times, September 25, 2003.
  7. Gilda Grard, Mélanie Caron, Illich Manfred Mombo, Dieudonné Nkoghe, Statiana Mboui Ondo, Davy Jiolle, Didier Fontenille, Christophe Paupy, and Eric Maurice Leroy. “Zika Virus in Gabon (Central Africa)–2007: A New Threat from Aedes albopictus?” PLOS Neglected Tropical Diseases 8, no. 2 (2014): e2681.
  8. Constância F.J. Ayres. “Identification of Zika Virus Vectors and Implications for Control.” The Lancet Infectious Diseases 16, no. 3 (2016): 278–279.
  9. Centers for Disease Control and Prevention. “HAI Data and Statistics.” 2016. Accessed Jan 2, 2016.

Learning Objectives

Humans play host to tons of disease-causing agents: viruses, bacteria, fungi, and other pathogens. The World Health Organization defines infectious disease as disease caused by pathogenic microorganisms that “can be spread, directly or indirectly, from one person to another.” The disease agents are simply using the host as a habitat for their own survival and reproduction, causing infection in the host. Infectious diseases include ringworm, the flu, HIV, and Zika.

Diseases spread through host-to-host contact or through vector-to-host contact. Imagine you have the flu, and you sneeze, making a sneeze cloud full of small droplets of liquid, each of which contains the flu virus. If those droplets make their way to the mucus membranes, like the nose or mouth, of another potential host, the disease can spread to them. Some diseases require exchange of bodily fluids (HIV, STIs), while others need direct contact with an individual or object that carries the disease agent (scabies, lice), and others can spread in droplets in the air (flu, cold).

Agencies like the CDC devote many research dollars and personnel to predicting infectious disease spread. They oversee three National Centers for:

  • Emerging and Zoonotic Infectious Diseases (NCEZID) – Zika, etc.
  • HIV/AIDS, Viral Hepatitis, STD, and TB Prevention (NCHHSTP) – blood-born and sexually transmitted diseases
  • Immunization and Respiratory Diseases (NCIRD) – influenza, etc.

At those agencies biologists who specialize in mathematical modeling use data and math to try to predict how a disease will spread. These predictions help public health agencies determine if they have an epidemic situation of disease spread, how many people will become sick, how fast the disease will spread, and what actions a local community might take to prevent that spread.


The roots of the basic reproduction concept can be traced through the work of Ronald Ross, Alfred Lotka and others, [30] but its first modern application in epidemiology was by George Macdonald in 1952, [31] who constructed population models of the spread of malaria. In his work he called the quantity basic reproduction rate and denoted it by Z 0 > . Calling the quantity a rate can be misleading, insofar as "rate" can then be misinterpreted as a number per unit of time. "Number" or "ratio" is now preferred. [ citation needed ]

Contact rate and infectious period Edit

With varying latent periods Edit

Latent period is the transition time between contagion event and disease manifestation. In cases of diseases with varying latent periods, the basic reproduction number can be calculated as the sum of the reproduction numbers for each transition time into the disease. An example of this is tuberculosis (TB). Blower and coauthors calculated from a simple model of TB the following reproduction number: [33]

Heterogeneous populations Edit

The basic reproduction number can be estimated through examining detailed transmission chains or through genomic sequencing. However, it is most frequently calculated using epidemiological models. [36] During an epidemic, typically the number of diagnosed infections N ( t ) over time t is known. In the early stages of an epidemic, growth is exponential, with a logarithmic growth rate

Simple model Edit

Latent infectious period, isolation after diagnosis Edit

In this model, an individual infection has the following stages:

  1. Exposed: an individual is infected, but has no symptoms and does not yet infect others. The average duration of the exposed state is τ E > .
  2. Latent infectious: an individual is infected, has no symptoms, but does infect others. The average duration of the latent infectious state is τ I > . The individual infects R 0 > other individuals during this period. after diagnosis: measures are taken to prevent further infections, for example by isolating the infected person.

Although R 0 > cannot be modified through vaccination or other changes in population susceptibility, it can vary based on a number of biological, sociobehavioral, and environmental factors. [28] It can also be modified by physical distancing and other public policy or social interventions, [43] [28] although some historical definitions exclude any deliberate intervention in reducing disease transmission, including nonpharmacological interventions. [24] And indeed, whether nonpharmacological interventions are included in R 0 > often depends on the paper, disease, and what if any intervention is being studied. [28] This creates some confusion, because R 0 > is not a constant whereas most mathematical parameters with "nought" subscripts are constants.

Methods used to calculate R 0 > include the survival function, rearranging the largest eigenvalue of the Jacobian matrix, the next-generation method, [46] calculations from the intrinsic growth rate, [47] existence of the endemic equilibrium, the number of susceptibles at the endemic equilibrium, the average age of infection [48] and the final size equation. Few of these methods agree with one another, even when starting with the same system of differential equations. [42] Even fewer actually calculate the average number of secondary infections. Since R 0 > is rarely observed in the field and is usually calculated via a mathematical model, this severely limits its usefulness. [49]

History of Germ Theory

Early Theories of Disease

In ancient Greece, it was thought that disease was spread not via direct contact with other infected individuals, but rather via infectious “seeds” in the air or food products. Furthermore, such seeds could reside within an individual’s body, causing a subsequent relapse of disease at a later time. This concept was later revisited by scholars in the Middle ages (e.g., Girolamo Fracastoro), who added that disease could be caused by direct or indirect contact, as well as via long distances. The idea that the disease-causing seeds could lie dormant was also further reaffirmed, and many diseases were categorized based on the length of dormancy.

  • Jar 1: Meatloaf and an egg exposed to the air without a lid.
    Results: Maggots covering the egg and meatloaf.
  • Jar 2: Meatloaf and an egg tightly sealed with a lid.
    Results: No maggots.
  • Jar 3: Meatloaf and an egg with no lid, but the jar was covered with gauze.
    Results: Maggots on top of the gauze.

Based on these findings, Redi concluded that the maggots were only found on accessible surfaces and thus, refuted spontaneous generation.

Another early microbiologist from the 1600’s was Anton van Leeuwenhoek, who was the first to directly observe the presence of microorganisms (which he referred to as “animalcules”) through his invention of the first microscope. The notion that disease was caused by creatures that could be visualized only with a microscope was later postulated by Richard Bradley in the 1700’s. This theory was later supported by Marcus Antonius von Plenciz, who wrote a book describing that the diseases caused by microscopic organisms could be further classified into those that were contagious but did not cause epidemics, and those that exhibited both qualities. Von Plenciz further described the ubiquitous presence of microscopic organisms.

Miasma Theory

The predominant theory until germ theory of disease was eventually accepted in the 19th century was termed “miasma theory”, meaning “pollution” or “bad air”. Miasma theory stipulated that disease originated from the decomposition of organic matter, causing a noxious vapor harboring disease-causing agents. Moreover, individuals could contract disease by inhaling foul-smelling air associated with contaminated drinking water, unsanitary conditions, and air pollution.

Non-Communicable Diseases and Communicable Diseases

Normally heart, lungs and diseases of central nervous system occur due to insufficient growth of organs.

(ii) Deficiency Diseases:

These occur due to malnutrition.

Allergies occur due to hypersensitivity to foreign substances.

It occurs due to uncontrolled growth of tissues.

(v) Diseases by Agents:

These diseases occur due to physical agents like, heat, cold etc.

(vi) Diseases by Mechanical Agents:

These diseases occur due to mechanical factors like injury, friction etc.

(vii) Metabolic Diseases:

They occur due to metabolic disorders.

According to Sir Francis Galton (1883, Father of Eugenics), it is the, “study of agencies under social control that may improve or impair the racial qualities of future generation either mentally or physically.”

Improvement of human race can be done either by:

(i) Encouraging desirable genetic qualities by breeding.

(ii) Suppressing harmful genes preventing such breeding.

It is the therapeutic treatment to suppress the genetic diseases. From amnio-centesis nearly 30 genetic disorders can be revealed. So these defective types of foetus can be aborted.

2. Communicable Diseases:

These diseases are mainly caused by different organisms. They spread in a society. So they are called as communicable or infectious diseases. These diseases spread through food, water, air, touch and blood. Some of these diseases are highly epidemic in nature. Therefore it is of great concern to the society.

The communicable diseases can be studied under three aspects:

Even before the invention of microscope the involvement of micro-organism in the disease was suspected. Louis Pasteur and Robert Koch put forwarded the germ theory of the diseases. Pasteur demonstrated that the micro-organisms are responsible for many diseases in animals. Later Robert Koch isolated the bacteria, causing the disease, anthrax. He identified a set of four conditions that had to be satisfied before a particular pathogen is attributed to a disease.

Koch’s Postulations:

1. The organisms must be regularly found in the animals that have the disease.

2. It must be isolated and grown in pure culture on artificial media.

3. When a healthy animal is inoculated with this culture, it must develop the disease and show the characteristic symptoms.

4. The same organism must be recovered from the inoculated animal.

Applying Koch’s postulations causative organisms of tuberculosis, cholera, typhoid and diphtheria were identified. Later diseases like chickenpox, small pox, and measles were found to be caused by viruses. Gradually other agents like, protozoa, helminthes, were identified as disease causing organisms. Parasitology was a boon to medical science. It helped to treat different diseases. Parasitology revealed the identification of causative organisms, life cycle, host range, and adaptation, which helped in control and eradication of the disease.

It deals with the cause of spread of infectious diseases in the society. John Snow for the first time traced the epidemiology of cholera. Later on epidemiology of other diseases were traced out. It helped to check the spread of disease in a society.

It deals with the self defence mechanism of the body. Edward Jenner (1742-1823), an English doctor noticed that dairymen and milk maids hardly suffer from small pox. He observed that they suffer from a mild disease called cow pox, infected from their cows.

Jenner collected some fluid from the sore, caused by cowpox and injected it to a healthy boy named James Phipps on 14th May, 1776. The boy suffered from cowpox and it healed after some days. After two months, Jenner boldly injected small pox fluid into James Phipps, and the germs could not induce the disease even after repeated inoculation. This led Jenner to prepare vaccines for small pox.

Immunity of our body is caused by the production of antibody developed in response to antigen. Any foreign germ when enters into the body produces antigen. The body produces specific antibodies to a particular antigen. The antibodies destroy the antigens. The understanding of immunity has helped the scientist s to prepare vaccines against many diseases.


Berkow, R. The Merck Manual of Diagnosis and Therapy, 16th ed. Rahway, NJ: Merck Research Laboratories, 1992.

Carlson, K. J., S. A. Eisenstat, and T. Ziparyn. The Harvard Guide to Women's Health. Cambridge, MA: Harvard University Press, 1996.

Center for Disease Control, Division of Sexually Transmitted Diseases. <> .

Madigan, Michael T., John M. Martinko, and Jack Parker. Brock Biology of Microorganisms, 9th ed. Upper Saddle River, NJ: Prentice Hall, 2000.

Sharing Bodily Fluids

During this lab you will share “bodily fluids” with other students in the lab to simulate the spread of an infectious disease through a population.


  1. Obtain a numbered vial of solution and a plastic pipet from your instructor.
  2. Record your student name and vial number on the class data sheet.
  3. Share bodily fluids with another person in lab. Use the plastic pipet to withdraw solution from your vial and place 5 drops of your solution in another classmate’s vial. Your classmate will also share fluids with you in the same way. Return the cap to the vial and invert to mix.
  4. Record the name of the person you shared bodily fluids with in the table below.
  5. Exchange bodily fluids with another person following the directions above. Record the name of the person with whom you exchanged fluids.
  6. Exchange fluids with another student (different than the first two) and record his/her name below. You should complete three total fluid exchanges.

My vial #:_______________________

Record of Bodily Fluid Exchanges

Exchange 1: ______________________________
Exchange 2: ______________________________
Exchange 3: ______________________________

Your lab instructor will add a drop of the test reagent to determine if you are infected with the disease. If your sample turns pink then you are infected. If it turns yellow you are not infected. If you are positive for the disease you may have originally had the disease or you may have contracted the disease in lab today from sharing bodily fluids.

  • Are you infected?
  • Is it possible to determine if you were originally infected or did you contract the disease from someone during today’s lab?

As a class you will fill in Table 1 below. Include each person’s name. If your test result is positive put a plus sign (+) next to your name. If your result is negative put a negative sign (−) by your name. For those individuals that are positive, record whom they exchange fluids with and whether that person was positive or negative.


  1. How many people in the class are infected?
  2. Can you determine who was originally infected?
  3. If you can, whom do you think was originally infected?
  4. What do the class results show about the spread of disease through activities in which bodily fluids are shared?

Fill out a table similar to Table 1 for all members of your class. Be sure to add as many tables as there are students!

Table 1. Class results for bodily fluid exchange activity
Student’s Name Test result (+/−) Exchange #1 (+/−) Exchange #2 (+/−) Exchange #3 (+/−)

After you’ve completed Table 1, discuss with your lab group how this experiment simulates a real life infection through a population and answer the following questions.

Small-Bowel Neoplasms

The mucosa of the small intestine encompasses about 90% of the luminal surface area of the digestive system, but accounts for only 2% of GI malignant neoplasms. Neoplastic processes with a propensity for the ileum are adenocarcinoma, lymphoma, and carcinoid tumor. Small-bowel neoplasms may occur sporadically, in association with genetic diseases (familial adenomatous polyposis coli, hereditary nonpolyposis colorectal cancer, or Peutz–Jeghers syndrome), or in association with chronic intestinal inflammatory disorders (CD or celiac sprue).

The diagnosis is often made late because of their uncommon occurrence and nonspecific symptoms. The CT appearance of ileal adenocarcinoma is an annular and constricting lesion involving a short segment of bowel. For patients with carcinoid, CT reveals an ill-defined spiculated mass with a stellate pattern containing calcification. Lymphoma most commonly manifests as single or multiple segmental areas of markedly thickened (1.5𠄷 cm) circumferential thickening, or may ulcerate with formation of a fistulous tract to adjacent bowel loops, mimicking CD [34].

The risk of GI cancer is elevated in patients with IBD (eg, 60-fold higher compared to the general population). Small-bowel adenocarcinoma complicating CD is predominantly seen in men, in patients with excluded bowel loops, and at the distal ileum in an area of active disease [35]. Carcinoid tumors have also been described in association with CD, and these cases tend to be malignant and have a worse prognosis [36]. Ileal carcinoid tumor should be suspected in elderly CD patients presenting with obstructive symptoms. In general, the presence of suspected ileal CD refractory to medical therapy should alert clinicians to the possibility of a small-bowel neoplasm. When technically feasible, ileoscopy with biopsy may help distinguish between the two and guide early diagnosis and treatment.

Communicable Diseases

Overcrowding, poor ventilation, poor health, poor diets, homelessness and living and working with people who have migrated from areas where disease is more common, all may affect the likelihood of catching a disease.

Indirect Transmission

Some pathogens, like the protoctista plasmodium that causes malaria, use vectors for transmission.

Indirect transmission of plant pathogen occurs as a result of insect attack.

The fungus that causes Dutch elm disease is carried by the beetle Scolytus multistriatus

Plant Defences against pathogens

  • Physical Defences
  1. Cellulose cell wall – this acts as a physical barrier but also contains many chemical defences that can be activated when a pathogen is detected
  2. Thickening of the cell wall with lignin – lignin is a phenolic compound and completely waterproof as well as largely indigestible
  3. Waxy cuticles – these prevent water collecting on the cell surfaces which removes the water that the pathogenic cells need to survive
  4. Bark – most back also contains a variety of chemical defences as well as being a physical barrier to disease
  5. Stomatal closure- stomata are possible points of entry for pathogens and so the guard cells can close them when pathogenic organisms are detected
  6. Callose – callose is a large polysaccharide that is deposited in the sieve tubes at the end of the growing season around the sieve plates and blocks the flow in the tube. This can prevent a pathogen spreading around the plant
  7. Tylose formation – tylose is a balloon like swelling or projection that fills the xylem vessel, when fully formed it can completely block off that part of the xylem vessel. It also contains a high concentration of chemicals such as terpenes that are toxic to pathogens
  • Chemical Defences
  1. Plant tissues contain a variety of chemicals that have anti-pathogenic properties including terpenoids, phenols, alkaloids and hydrolytic enzymes.
  2. Some of these chemicals such as the terpenes in tyloses and the tannins in bark are present before infection. However, because the production of chemicals requires a lot of energy, many chemicals are not produce until after an infection is discovered.

  • Cell walls become thickened and strengthened with additional cellulose
  • Deposition of callose between the plant cell wall and cell membrane near the pathogen. Callose deposits impeded cellular penetration at site of infection, strengthen the cell wall and block of plasmodesmata.
  • Oxidative bursts that produce highly reactive oxygen molecules capable of damaging the cells of invading pathogens
  • An increase in the production of chemicals
  • Necrosis – deliberate cell suicide
  • Canker – a sunken necrotic lesion in the woody tissue such as the main stem or branch that causes the death of the cambium tissue in the bark.

Primary Defences against Disease

Primary defences are the defences in place that prevent pathogenic material from entering the body.

The skin is the main primary defence. The outer layer of skin is called the epidermis and consists of layers of keratinocytes. The keratinocytes are produces at the base of the epidermis and migrate out to the surface of the skin, slowly drying out and their cytoplasm in replaced by the protein keratin in the process of keratinisation. By the time the cells reach the surface they are dead. The layer of dead keratinised cells act as effective layer of disease prevention.

Blood Clotting and skin repair

  1. Abrasions or lacerations damage the skin and open the body to infection
  2. The body prevents excess blood loss by forming a clot and making a temporary seal to prevent infection
  3. Calcium ions and 12 clotting factors are released from the platelets and damaged tissue
  4. Damage to blood vessel exposes collagen
  5. Platelets bind to collagen fibres and release clotting factors, a temporary plug is formed
  6. Inactive thrombokinase in blood (factor X) is turned into active thrombokinase (an enzyme)
  7. Prothrombin in blood and thrombonkinase and Ca2+ ions make active thrombin
  8. Active thrombin turns the soluble fibrinogen in the plasma into insoluble fibrin which attach to the platelets in the plug and clot, trapping more red blood cells and platelets.
  9. As the skin grows and the scab shrinks the edges of the laceration are pulled together.

Mucous Membranes

The epithelial layer contains mucus-secreting cells called goblet cells and also mucus secreting glands under the epithelium.

The mucus traps any pathogens that may be in the air

The epithelium is also ciliated

Cilia are tiny hair-like organelles that can move in a coordinated fashion to waft the mucus along.

Coughing and sneezing – areas that are prone to microorganism attack are sensitive and respond to irritations by coughing sneezing and vomiting in the hope that the expulsion of air will propel the pathogen from the body.

Inflamation – the tissue may be hot and painful as the presence of harmful microorganisms has been detected by mast cells which release a cell signalling substance called histamine which causes vasodilation to make the capillary walls more permeable to white blood cells and proteins. The increased production of tissue fluid causes the swelling (oedema)

Eyes are protected by antibodies and enzymes in tear fluid

The ear canal is lined with wax

The female reproductive system is protected by a mucus plug in the cervix and by maintaining relatively acidic conditions in the vagina.

Specific Immune Response

Antibodies – specific proteins released by plasma cells that can attach to pathogenic antigens

Clonal expansion – an increase in the number of cells by mitotic cell division

Interleukins – signalling molecules that are used to communicate

Specific immune response involves B lymphocytes (B cells) and T lymphocytes (T cells) which are white blood cells with specialised receptors on their cell surface membranes. Antibodies are produced by B lymphocytes, and these neutralise foreign antigens. Long term disease protection is provided. An immunological memory is produced as B memory cells are released and circulate in the body for a number of years.

  1. Pathogen enters the body
  2. The antigens on the pathogen are presented on the pathogens cell membrane as it travels in the body fluids, on infected cells, and on the plasma membrane of macrophages that have engulfed the pathogens during the secondary non-specific response.
  3. T cells (from thymus) and B cells (from bone marrow) must detect the antigen from one of these three sources
  4. The detection of the pathogenic antibodies triggers clonal expansion in both T and B cells.
  5. T helper cells release cytokines that further stimulate the development of B cells
  6. Proliferation occurs once the correct lymphocytes have been activated.
  7. Cells differentiate and T & B memory cells are produced to remain in the blood should the body ever come under attack from the same pathogen again.
  8. T killer cells attack infected host cells and plasma cells make antibodies (the differentiation for this is triggered by cytokines from the macrophages
  9. Finally T regulator cells end the immune response to prevent the attack of the body’s own cells.

T lymphocytes

Come from the bone marrow and develop in the thymus

T helper cells (Th) – release cell signalling molecules (cytokines) that stimulate the immune response of B cells to develop and stimulate phagocytosis by phagocytes

T killer cells (Tk) – attack and kill host-body cells that display the foreign antigen as well as infected body cells

T memory cells (Tm) – provide long term immunity by staying in the blood for a long time

T regulator cells (Tr) – inhibit and end the immune response, preventing autoimmunity

They are involved in cell-mediated response (combat microorganisms)

They are a complementary shape to the antigen of pathogens and once the T cell has found a complementary antigen clonal expansion takes place produced by mitosis

B lymphocytes

Grow completely in the bond marrow

Plasma cells – derived from the B lymphocytes, these cells manufacture antibodies

B memory cells – cells that remain in the blood for a long time, providing long-term immunity

They are involved in the humoral response (producing antibodies)

Cell signalling

Macrophages release monokines which attract neutrophils (by chemotaxis – the movement of cells towards a particular chemical) and stimulate differentiation of B cells (and the release of antibodies)

T cells and macrophages release interleukins which stimulate clonal expansion (proliferation) and the differentiation of B & T cells

Many cells release interferon which inhibits virus replication and stimulates T killer cells

Autoimmune diseases

A disease that occurs when the immune system attacks a part of the body

Arthritis – a painful inflammation of a joint that starts with antibodies attacking the membranes around the joint

Lupus – swelling and pain in any part of the body, antibodies attack certain proteins in the nucleus of cells and affected tissue

Antigens are molecules that stimulate an immune response, usually proteins/glycoproteins in the pathogen’s plasma membrane, and when detected the production of antibodies is commenced.

Antibodies are specific to the antigen as antigens are specific to the organism. Our own antigens are recognised as ‘self’ by the immune system and do not provoke a response

Antibodies are immunoglobins (complex proteins produces by the plasma cells) and are released in response to an infection, they have a region with a specific shape to the antigen, antibodies attach to antigens and render them harmless

4x polypeptide chain, 2x light chains & 2x heavy chains

The tips of the y are the variable region but is the same for every type of antibody

A group of antibodies that bind to pathogen antigens and then act as binding sites for phagocytic cells

Some are produced as part of a specific immune response and bind to specific antigens

Antibodies flag up a pathogen for the phagocyte/attach to antigen which has a use to the pathogen, disabling it.

They also prevent the pathogen to enter the host cell.

Neutralise pathogens that use their antigens to bind to host cells etc.

By attaching onto the pathogen they make them easier to identify and easier for the phagocytes to bind to them/engluf them.

Antibodies that cause the pathogens to stick together (agglutinate) by making crosslinks between their antigens

This makes the pathogen non-effective and easily phagocytosed

Some antibodies bind to molecules that are release by pathogenic cells. These molecules may be toxic and the action of anti-toxins render them harmless.

Primary and Secondary Responses

Primary immune response – initial response caused by a first infection
Secondary immune response – more rapid and vigorous response caused by a second or subsequent infection by the same pathogen

  1. infection by pathogen
  2. lag phase
  3. antibodies produced
  4. antibody level rises to combat infections
  5. pathogen dealt with
  6. antibody level declines – short lived
  7. secondary immune response is much faster


Vaccination – a way of stimulating an immune response so that immunity is achieved, provides immunity to specific disease by deliberate exposure to a weakened/dead strain of antigenic material

Antigenic material takes many forms:

  • Whole live microorganisms (usually not very harmful g. smallpox which prevents cowpox virus, a much nastier disease)
  • Harmless attenuated version of the pathogenic organism e.g. measles & TB
  • Dead pathogen e.g. typhoid
  • Antigen preparations only, no actual pathogen e.g. hep B
  • Toxoids – harmless version of a toxin e.g. tetanus
  • Herd vaccination – providing the vaccine to all or almost all of the population so that the pathogen cannot spread, it is necessary to vaccinate 80 – 95% of the population to completely immunise the population
  • In the UK young children are immunised against the following diseases: diphtheria, tetanus, whooping cough, polio, meningitis, measles, mumps and rubella.
  • Ring vaccination – used when a new disease case is reported, vaccinated all people in the immediate vicinity of the case, also used to control livestock disease
  • Epidemic – a rapid spread of disease through a high proportion of the population
  • Influenza – a killer disease caused by a virus, people aged 65+ are most at risk as well as people with respiratory tract problems, the swine flu pandemic is an example of this virus

Types of immunity

Active immunity – immune system activated and own antibodies manufactured

Artificial immunity – immunity achieved as a result of medical intervention

Natural immunity – immunity achieved through normal life processes
Passive immunity – immunity achieved when antibodies are passed to the individual through breast feeding or injection

Development of Drugs

Antibiotic – a chemical which prevents the growth of microorganisms, can be antibacterial or antifungal

Personalised medicine – development of designer medicines for individuals

Synthetic biology – re-engineering of biology, from the production of new molecules that mimic a natural process to the use of natural molecules to produce new biological systems that do not exist in nature

The antibiotic penicillin was discovered by Alexander Fleming accidentally.

Traditional remedies

Morphine originated in the use of sap from unripe poppy seed heads in Neolithic times, in the 12 th century the opium from poppies was used as an anaesthetic and by the 19 th century morphine and opium were used to reduce nervous action in the central nervous system

Willow bark is used to relieve pain and fever. Its active ingredient was found to reduce the side effect of stomach bleeding by adding an acetyl group which lead to the development of aspirin and ibuprofen

Wildlife remedies

Monkeys, bears and other animals rub citrus oils on their coats as insecticides and antiseptics to prevent insect bites and infection. Birds line their nests with medicinal leaves to protect young from blood sucking mites. Chimps swallow leaves folded in a particular way to remove parasites from the digestive tract

Further plant research

Scientists use the traditional plant medicines as a starting point for new medicines and then try to isolate their active ingredient.

Pharmaceutical companies also research the way that microorganisms cause disease so that they can model ideal proteins and glycoproteins to act as medicines on these drugs. E.g. the HIV virus binds to the CD4 and CCR5 receptors on T helper cells. In order to cure this scientists are experimenting with blocking this binding with other similarly shaped proteins generated in computer models.

Overuse and misuse of antibiotics have enabled microorganisms to develop resistance which limits the effectiveness of current medicines.

Compare the adaptations of different pathogens that facilitate their entry into and transmission between hosts


  • Adaptation: Parasite developed a mutation which made it resistant to chloroquine (antimalarial drug).
  • Adaptation: Uses a host (mosquitos) to penetrate skin (physical barrier) and transmit disease.


  • Adaptation: Uses chemotaxis to move through mucus (barrier) and transmit disease.
  • Adaptation: Can change to pH of surrounding environment to survive in organism.

Watch the video: What is cystic fibrosis? Animation (July 2022).


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