What Factors Comprise the Epidemiologic Triangle?

The Centers for Disease Control and Prevention (CDC) define epidemiology as the study of the distribution and determinants of health-related states and events in specified populations.¹ To complete this study of “health-related states and events”, scientists use a tool called the epidemiologic triangle, or the epidemiologic triad.

The epidemiologic triangle is made up of three components that contribute to the spread of disease: an external agent, a host and an environment in which the agent and host meet. Between the vertices, scientists will often describe the center of the triangle as representing time. Although a number of models of disease causation exist, the triangle model is considered the simplest way to understand the factors behind infectious disease and other public health issues. Another way to think of the triangle model is the what (the agent), who (host), where (environment) and when (time) of health issues.³

The CDC describes the three components in the following ways:

The agent is the cause of the health event. When it comes to infectious disease the agent is a microbe or what people typically think of as germs. As epidemiology has evolved to cover more public health issues beyond diseases, agents can also be represented as physical or chemical factors.²

Epidemiologic triangle agents include:³

• Bacteria: Bacteria are single celled organisms that are able to reproduce on their own. Commonly-known bacteria are streptococcus (which causes strep throat), lyme disease, syphilis and anthrax.⁴
• Viruses: Viruses are made up of genetic material but cannot reproduce on their own and instead 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 are commonly found in water. Malaria is an example of a parasitic protozoan.
• 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.⁷
• Physical Forces: In the scientific sense physical forces that could be responsible for public health events include environmental forces like extreme temperatures, vibration or noise. Other physical forces are more minute like carpal tunnel from repetitive movements.⁷

It’s important to note that the epidemiologic triad is a useful tool for many diseases and health events but the model only works for those issues that have one single necessary cause. Therefore, it isn’t an adequate model to use for diseases such as cardiovascular disease, cancer, and other diseases that appear to have multiple contributing causes.³

The host is the organism which is exposed to and harbors a disease. 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. The host also presents symptoms of a disease or health issue. As the CDC explains, a variety of factors intrinsic to the host, sometimes called risk factors, can influence an individual’s exposure, susceptibility, or response to a causative agent.^2,3

Risk factors include:²

• Opportunities for exposure: Exposure factors often fall into the category of lifestyle choices including sexual practices, hygiene, and other personal choices as well as by age and sex.
• Susceptibility and response: How likely you’ll be impacted and how a disease or health event will affect someone include factors like genetics, nutritional and immunologic status, anatomic structure, presence of disease or medications, and psychological makeup.

Completing the triangle used by epidemiology to model the study of disease and health issues are the extrinsic factors that affect the agent and provide opportunity for the host to be exposed. The environment represents the favorable conditions for an agent to cause a health event. Environmental factors include physical features like geology or climate, biologic factors like the presence of disease transmitting insects and socioeconomic factors like crowding, sanitation and access to health services. For water-dwelling protozoa, that could mean a body of dirty water, E. Coli need warm temperatures to survive and the flu and colds thrive in environments with people in close proximity like college dormitories in the winter months of North America.^2,3

In the center of the triangle is time. This factor is less consistently used to represent one thing. Time can represent the incubation period of a disease (the time between when the host is infected and when symptoms start to present), duration of the illness or amount of time a person can be sick before the disease has run its course and results in death or recovery. It also can be used to describe the period from an infection to the threshold of an epidemic for a population.³

If the triangle represents the who, what and where of infectious disease and health issues, epidemiologists and public health workers are responsible for determining the balance of the factors, or the why, and how to disrupt or break the triangle to stop the issue from progressing.

The best way to understand this model is to look at some examples of how it’s been used.

The National Institute for Health has broken down how the triangle can be applied to COVID-19.⁸

First epidemiologists have to identify the three vertices of the triangle:

Agent: A type of virus called a coronavirus, identified as SARS-CoV-2.

Host: Humans, especially those with comorbidities, and older adults.

Environment: SARS-CoV-2 spreads through droplets in the air and on surfaces.

The next step to investigate potential interrupting factors to break the sides of the triangle, or whatever connects each of the vertices.

Agent-Host Interrupting Factors for COVID-19: These are anything that can keep an agent from infecting a host. For COVID-19 the way to interrupt the virus from infecting the host is to get vaccinated.

Environment-Host Interrupting Factors for COVID-19: These interrupting tactics will sound familiar. Most environment-host interruptions are common public health measures to prevent situations in which hosts are likely to be infected. Some of these factors for COVID-19 include sheltering in place, social distancing, hand washing and disinfecting surfaces. In hospitals this also meant temporarily cancelling elective surgeries at hospitals to prevent unnecessary exposure of hosts to an environment where COVID was likely to be.

Agent-Environment Interrupting Factors for COVID-19: These tactics will also sound familiar. These are all the ways scientists have recommended to prevent the agent, the COVID-19 virus, from spreading in different environments. These tactics include self-quarantine of infected individuals, travel restrictions, mask wearing and frequent disinfection of surfaces.

Scientists were also able to determine the center of the triangle, the factor of time, that shows the COVID virus spreads fairly quickly and hosts are generally capable of spreading the virus for two weeks after infection.

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