What Is the Epidemiologic Triangle?

For an epidemiologist, understanding how diseases spread is essential. Epidemiologists must learn about diseases and what makes populations susceptible to them, how they move through populations, and what allows them to survive and thrive. In this way, epidemiologists effectively study and fight the spread of disease.

Epidemiologists use a tool to help understand infectious disease known as the epidemiologic triangle. The epidemiologic triangle is a model for explaining the organism causing the disease and the conditions that allow it to reproduce and spread.

Blue triangle representing the epidemiologic triangle with sides labeled as host, agent, and environment.

The Epidemiologic Triangle

The Centers for Disease Control and Prevention (CDC) defines an outbreak or epidemic as “the occurrence of more cases of disease, injury, or other health condition than expected in a given area or among a specific group of persons during a specific period.” When investigating how a disease spreads and how to combat it, the epidemiologic triangle can be an invaluable tool. The epidemiologic triangle is made up of three parts: agent, host and environment.


The agent is the microorganism that actually causes the disease in question. An agent could be some form of bacteria, virus, fungus, or parasite.


The agent infects the host, which is the organism that carries the disease. A host doesn’t necessarily get sick; hosts can act as carriers for an agent without displaying any outward symptoms of the disease. Hosts get sick or carry an agent because some part of their physiology is hospitable or attractive to the agent.


Outside factors can affect an epidemiologic outbreak as well; collectively these are referred to as the environment. The environment includes any factors that affect the spread of the disease but are not directly a part of the agent or the host. For example, the temperature in a given location might affect an agent’s ability to thrive, as might the quality of drinking water or the accessibility of adequate medical facilities.

Epidemiologic Triangle Examples

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


The human immunodeficiency virus (HIV) epidemic reached its height in 1985, when there were roughly 130,000 annual cases of infection. Since that time, reported annual infections have decreased dramatically, though the CDC estimates that there are currently around 1.2 million people in the United States living with HIV. By better understanding how HIV is communicated, epidemiologists were able to make progress on treatment and prevention.




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 a 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 socioeconomic 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.

Smoking-Related Diseases

The CDC estimates that more than 480,000 people die from smoking-related illnesses every year. Life expectancy tends to be shorter for those who smoke, and smoking-related diseases are considered to be the leading preventable cause of death in the United States. CNN reports that smoking rates are on the decline; this is due to a variety of anti-smoking efforts.




Unlike the agent in many epidemiologic triangles, cigarettes and other forms of tobacco are not microbes, and smoking is not contagious in the usual sense. It is not transmitted from person to person, though second-hand smoke is dangerous. However, a carcinogen in the smoke of a cigarette is an agent. People become a potential host for smoking-related diseases when they smoke cigarettes or when they inhale second-hand smoke. A host can then suffer a variety of ailments, from lung cancer to heart disease to diabetes. Not all people who smoke suffer the same effects at the same rates; genetics play a part here, as do environmental factors. The reasons people smoke are almost entirely social. Pressure from peers or friends, a need to fit in, or susceptibility to the marketing efforts of tobacco companies are all factors that can lead to smoking and, consequently, smoking-related diseases. In addition, environmental factors such as the frequency at which people smoke and the length of time they remain a smoker can affect their chances of contracting a smoking-related disease. Because cigarettes and other tobacco products are highly addictive, individuals are likely to continue to smoke once they start.

Learn More About Epidemiology

Those interested in becoming an epidemiologist should pursue an education in public health. Rivier University offers an online public health master’s degree and an online public health undergraduate degree, which can help pave the way to a master’s. Together, the two degrees create a foundation on which to build a career in epidemiology. Rivier’s online degree programs allow you to learn in a dynamic, flexible environment that supports your professional goals while enabling you to work full-time and maintain your personal commitments.