6.2: Epidemiologic Triangle
Like the early epidemiologists, let’s start by looking at infectious diseases. If we’re trying to understand a certain infectious disease, we need to know the factors that lead to a person’s contracting it. The epidemiologic triangle is a good visual representation of the different factors required for an infectious disease to spread. There has to be some sort of pathogen that causes the disease - this is called the “agent”. This agent is usually one of the following pathogens: bacteria, virus, fungi, protozoa, or prion (we’ll go over these pathogens more in another chapter). This agent has to have a host in order to cause disease in that host. Assuming we are worried only about human hosts (not animals at this point), their risk and severity of disease is influenced by many factors, including their age, sex, susceptibility, immune system function, and other diseases or conditions that they might already have. The third corner of the triangle is the environment, and how it may encourage or discourage disease transmission. If the disease is transmitted through a vector (like a flea, tick, or mosquito bite, or another animal bite), then a fourth factor could be added to the middle of the triangle. The epidemiological triangle helps to identify questions that need to be answered in order to help prevent the spread of infectious diseases. See Fig. \(\PageIndex{1}\) for some examples.