Breaking Down the Epidemiological Triangle: Key Factors Explained

The Centers for Disease Control and Prevention (CDC) defines epidemiology as the "study of the distribution and determinants of health-related states and events in specified populations."¹ To further examine how health conditions affect populations, scientists look at three factors that connect to one another in any health-related situation: the external agent, the host, and the environment where they meet. The intersections of these factors are referred to as the "epidemiological triangle.”

Understanding the epidemiological triangle model gives researchers, public health professionals and clinicians a clearer picture of how epidemiologists assess and mitigate health events.

In this article, we'll explore the epidemiological triangle in-depth and cover how you can apply it in your epidemiology career.

The epidemiological triangle is a framework for addressing the three major questions of a health event: what is causing it, who is affected, and where it is happening. In scientific terms, the “what” is the external agent that triggers a health effect; the “who” is the host upon whom the agent is acting; and the “where” is the environment in which the agent and host meet.

Scientists often describe the center of the triangle as representing the time between the vertices. That aspect gives health officials an answer to the question of when a health event occurs—both when it was initiated and what the future impacts may be.²

For example, in an outbreak of food poisoning, experts would find out who was affected (the hosts). Those people could answer questions about what they ate (the agent), where they procured it (the environment), and when it happened. That might reveal that all the affected people had a salmon dish from a certain restaurant on a particular night. Officials could determine where the restaurant bought the salmon and issue alerts to others who may have purchased it during the same time frame, preventing more people from eating it and getting sick.

The three sides of the epidemiological triangle are present in any health event, but nearly infinite permutations are possible. Epidemiologists closely examine the three components of the epidemiological triangle when assessing population-scale health events.

The agent is the cause of the health event. When it comes to infectious diseases, the agent is a microbe. As epidemiology has evolved to cover more public health issues beyond diseases, agents can also be physical or chemical factors.³

Epidemiological agents include the following:²

• Bacteria: Bacteria are single-celled organisms that are able to reproduce on their own. Strep throat and Lyme disease are examples of illnesses caused by bacteria.⁴
• Viruses: Viruses are made up of genetic material but cannot reproduce on their own. Instead, they infect cells with their DNA to be reproduced. The flu, the common cold, HIV and herpes are all types of viruses.⁵ 
• Fungi: Fungi are not plants because they cannot produce their own food and instead live off of other organisms, including plants, animals, and people. Fungal infections are often found in fingernails and toenails, on the skin as ringworm and in the vagina as yeast infections.⁶ 
• Protozoa: Protozoa are tiny parasites that live off other organisms and can cause disease. Malaria, for example, is caused by protozoa that are transmitted to humans via bites from mosquitoes that carry the parasite. Another common protozoan-caused condition is toxoplasmosis, which humans contract after exposure to infected cat feces.⁷ 
• Chemical Contaminants: Contaminants like poisonous metals such as lead or mercury, gases like chlorine and carbon dioxide, radiation and organic compounds like pesticides are all considered potential agents of health events or issues.⁸ 
• Physical Forces: Physical forces that could be responsible for public health events include forces that act upon people, such as extreme temperatures or weather events, along with the impacts of physical actions. For example, the physical training requirements of military service cause joint injuries in many veterans.⁹ 

The host is the organism that is exposed to and harbors disease and presents signs of illness. In epidemiology, the host is usually a human who gets sick but can also be an animal that acts as a carrier of disease but may or may not present illness.

In some cases, multiple hosts may carry an agent, and only certain hosts show health effects. For example, pigs can be hosts for the parasite Trichinella, which does not affect the animals. However, if humans eat undercooked pork that is host to Trichinella, they can become sick with trichinellosis.¹⁰

Generally speaking, the host who shows health effects from an agent is called the primary host. Asymptomatic carriers are secondary hosts. Some organisms find temporary hosts called paratenic hosts.

Completing the epidemiological triangle are the extrinsic factors that affect the agent and provide an opportunity for the host to be exposed. These factors include elements of the natural and built environment, such as the following:

• Geographic location
• Climate
• Presence of air pollutants
• Biologic factors like the presence of disease-transmitting insects
• Population density
• Sanitation
• Access to health services
• Public safety

The health effects of environmental factors like lack of sanitation are apparent in Gaza, where water treatment capabilities were drastically reduced at the start of the war with Israel in 2023. In the first month of the war, the World Health Organization reported over 15,000 cases of diarrhea in children in Gaza, a sharp increase over the typical 2,000 cases per month in the two years prior to the war.¹¹

In the center of the epidemiological triangle model is time. Time can represent the incubation period of a disease (the time between when the host is infected and when symptoms start to present), the duration of the illness or the amount of time a person can be sick before the disease has run its course and results in death or recovery.

Time may also refer to the period that a risk factor affects possible targets. For example, the risk of certain cancers increases with age. Breast cancer is most common in women and even more common in women over the age of 40. As a result, the screening guidance is different for women of different ages.¹²

The epidemiological triangle is a tool for clearly outlining a public health problem. Gathering answers to the nature of a health issue, which population it affects, and where they are exposed to the issue allows epidemiologists to see the big picture.

From that point, they can begin to develop a plan to stop the issue from progressing further. This may involve delivering effective therapies, implementing preventive measures and conducting public awareness campaigns to teach people how to protect themselves.

In 2025, officials in Texas reported an outbreak of measles in the state. The external agent was clear from the outset: Measles is an infectious illness, though it can be prevented with vaccination. Public health professionals and epidemiologists were able to determine the location of the cases through routine surveillance and investigate who was contracting the illness. They determined that a majority of those who had fallen ill were not adequately vaccinated.

With the three sides of the epidemiological triangle in place, public health professionals could make a recommendation to slow the outbreak. Authorities began recommending that at-risk individuals check their vaccine status and receive vaccines as needed.¹³

Novel health conditions present a more complex situation. In late 2019, public health officials reported significant cases of respiratory illness with high morbidity and mortality. To begin constructing the triangle, officials had to first identify the agent. They determined that it was a new strain of virus, which they labeled COVID-19.¹⁴

At the same time, they had to narrow down the population affected by the new infectious agent. By collecting demographic data on reported cases, they determined that all people were susceptible to infection, though severity was linked to underlying risk factors like age.

Finally, scientists saw that the spread of the virus was rapid. Further research into the virus confirmed that it was an airborne contagion, making any environment a potential site for transmission.

With a novel virus that affected all people, and no therapy or vaccine available initially, epidemiologists focused on reducing transmission. They recommended measures such as social distancing and isolating at home, hand washing, disinfecting surfaces, self-quarantine of infected individuals, travel restrictions, mask-wearing and frequent disinfection of surfaces. Once vaccines became available, experts recommended inoculation as a preventive measure.

The online Master of Science in Clinical Epidemiology at Kent State University can help you become a leader in your epidemiology career. Expert faculty will teach you clinical practices, analytical methods and regulatory guidelines necessary to conduct clinical research aimed at understanding diseases and other health problems and identifying effective prevention methods and treatments. To learn more about admission requirements for the program, schedule a call with an admissions outreach advisor today.