The components that contribute to the spread of a disease.
Deeper dive
To understand how an infectious disease spreads, public health officials ask questions reminiscent of journalists. They want to know: What was the agent that caused the disease? Who was the host? Where did the transmission occur? These three questions create the “Epidemiologic Triangle” model, which is used to determine the nature of an outbreak.
Question one: “What is the agent?” refers to the microbe — a bacteria, virus, fungi, or parasite — that is causing the disease. How did it invade, leave, or transmit to the host? Was the transmission direct (from a person coughing on another person)? Or indirect? (from eating contaminated food, or drinking dirty water?) Or through an animal or insect? (from the bite of a mosquito?)
Question two: “Who is the host?” refers to the human or animal that is exposed and harboring a disease. What was the risk, or the susceptibility of the person of getting the disease? The answers to the questions may be biological (do they have a genetic predisposition for the disease? Or a weak immune system?), behavioral (what are the person’s eating habits?), demographic or cultural (do they have access to clean water? Do they live on a farm?)
Question three: “Where did the transmission occur?” refers to the environment. The environment impacts the risk of an animal or person’s exposure to a pathogen. For example, what is the climate, geology, and habitat of the person or animal? Is the person living in a nursing home? What is the biological environment? Does the person live near a jungle or river, where there are mosquitos? And what is the person’s economic status? What is their occupation? Was there a natural disaster, like a hurricane, which caused mold to grow? Other environmental factors include the weather. In the winter, flu viruses spread more quickly, than in the warm summer months.
In the middle of the triangle is time. It is the period between exposure and signs of symptoms, which is called the incubation period. The time can provide information to epidemiologists about the nature of the pathogen, its source and identify those who were likely infected.
These are all questions journalists may ask public health officials as well when they are covering a disease outbreak and want to know more about what it means for their community.