Epidemiologic Triad- Agent, Host, Environment

  • The germ theory of disease is the currently accepted scientific theory for many diseases.
  • It states that microorganisms known as pathogens or “germs” can lead to disease.
  • However, the germ theory of disease has many limitations.
  • For example, it is well-known, that not everyone exposed to tuberculosis develops tuberculosis. The same exposure, however, in an undernourished or otherwise susceptible person may result in clinical disease.
  • Similarly, not everyone exposed to beta-hemolytic streptococci develops acute rheumatic fever.
  • There are other factors relating to the host and environment which are equally important to determine whether or not a disease will occur in the exposed host.

This demanded a broader concept of disease causation that synthesized the basic factors of agent, host, and environment focus on different classes of factors, especially with regard to infectious diseases.

  • The interaction and interdependence of agent, host, environment, and time are used in the investigation of diseases and epidemics.
  • The agent is the cause of disease;
  • The host is an organism, usually a human or an animal, that harbors the disease
  • The environments are those surroundings and conditions external to the human or animal that cause or allow disease transmission; and
  • time accounts for incubation periods, the life expectancy of the host or the pathogen, and duration of the course of illness or condition.
Epidemiologic Triad- Agent, Host, Environment

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The Agent

  • Agents of infectious diseases include bacteria, viruses, parasites, fungi, and molds.
  • With regard to non-infectious disease, disability, injury, or death, agents can include chemicals from dietary foods, tobacco smoke, solvents, radiation or heat, nutritional deficiencies, or other substances, such as poison.
  • One or several agents may contribute to illness.
  • Generally, the agent must be present for the disease to occur; however, the presence of that agent alone is not always sufficient to cause disease.
  • A variety of factors influence whether exposure to an organism will result in disease, including the organism’s pathogenicity (ability to cause disease) and dose.

The Host

  • A host offers subsistence and lodging for a pathogen and may or may not develop the disease.
  • The level of immunity, genetic makeup, level of exposure, state of health, and overall fitness of the host can determine the effect a disease organism will have on it.
  • Opportunities for exposure are often influenced by behaviors such as sexual practices, hygiene, and other personal choices as well as by age and sex. 

The Environment

  • The ability of the pathogen to accept the new environment can also be a determining factor because some pathogens thrive only under limited ideal conditions.
  • For example, many infectious disease agents can exist only in a limited temperature range.
  • Environmental factors can include the biological aspects as well as social, cultural, and physical aspects of the environment.
  • The surroundings in which a pathogen lives and the effect the surroundings have on it are a part of the environment.
  • The environment can be within a host or external to it in the community.

The Time

  • Time includes the severity of illness in relation to how long a person is infected or until the condition causes death or passes the threshold of danger towards recovery.
  • Delays in time from infection to when symptoms develop, duration of illness, and threshold of an epidemic in a population are time elements with which the epidemiologist is concerned.

The Epidemiologic Triad

  • A number of models of disease causation have been proposed.
  • Among the simplest of these is the epidemiologic triad or triangle, the traditional model for infectious disease.
  • The triad consists of an external agent, a susceptible host, and an environment that brings the host and agent together.
  • In this model, the disease results from the interaction between the agent and the susceptible host in an environment that supports the transmission of the agent from a source to that host.
  • While the epidemiologic triad serves as a useful model for many diseases, it has proven inadequate for cardiovascular disease, cancer, and other diseases that appear to have multiple contributing causes without a single necessary one.

The best way to understand the epidemiologic triangle is to see how epidemiologists use it to explain the spread of existing diseases.

Example- HIV AIDS


HIV is a viral infection that targets a person’s immune system, making it more vulnerable to other forms of infection. Because the virus targets the immune system itself, the body cannot effectively fight HIV on its own. HIV is communicated through direct contact with an infected person’s bodily fluids, and it primarily spreads through sexual contact or shared needles.


Scientists theorize that HIV was originally carried by chimpanzees and that humans who hunted these chimpanzees for meat became infected with a mutated form of the virus upon contact with the chimpanzees’ blood. HIV can be transmitted when bodily fluid such as blood comes into contact with a mucous membrane or damaged tissue (such as an open wound or the mucous membranes found inside the mouth).


There are a number of socio-economic factors that can impact the spread of HIV within a community. Communities with higher concentrations of sexually transmitted diseases and lower incidences of reporting — due to social pressure or otherwise — allow HIV to flourish. Poverty limits access to care and treatment, and discrimination can discourage individuals from being tested or seeking care.


  1. Park, K. (n.d.). Park’s textbook of preventive and social medicine.
  2. Gordis, L. (2014). Epidemiology (Fifth edition.). Philadelphia, PA: Elsevier Saunders.
  3. Hennekens CH, Buring JE. Epidemiology in Medicine, Lippincott Williams & Wilkins, 1987.
  4. White, F., Stallones, L., & Last, J. M. (2013). Global public health: Ecological foundations. New York, NY: Oxford University Press.
  5. https://www.cdc.gov/csels/dsepd/ss1978/lesson1/section8.html
  6. https://online.rivier.edu/epidemiologic-triangle/
  7. https://academic.oup.com/occmed/article/58/8/594/1467851
  8. https://slideplayer.com/slide/11979072/

About Author

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Sagar Aryal

Sagar Aryal is a microbiologist and a scientific blogger. He is doing his Ph.D. at the Central Department of Microbiology, Tribhuvan University, Kathmandu, Nepal. He was awarded the DAAD Research Grant to conduct part of his Ph.D. research work for two years (2019-2021) at Helmholtz-Institute for Pharmaceutical Research Saarland (HIPS), Saarbrucken, Germany. Sagar is interested in research on actinobacteria, myxobacteria, and natural products. He is the Research Head of the Department of Natural Products, Kathmandu Research Institute for Biological Sciences (KRIBS), Lalitpur, Nepal. Sagar has more than ten years of experience in blogging, content writing, and SEO. Sagar was awarded the SfAM Communications Award 2015: Professional Communicator Category from the Society for Applied Microbiology (Now: Applied Microbiology International), Cambridge, United Kingdom (UK). Sagar is also the ASM Young Ambassador to Nepal for the American Society for Microbiology since 2023 onwards.

3 thoughts on “Epidemiologic Triad- Agent, Host, Environment”

  1. other factors that increase the probability of of HIV transmission is raised viral load and also
    the co infection with other STIs.


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