Final answer:
An 'epidemic' is when a contagious disease affects a large number of people in a population based on the number of those exposed.
Step-by-step explanation:
When the number of people who get sick from a contagious disease depends on the number of people exposed, this is referred to as an epidemic. An epidemic occurs when a disease affects a greater number of individuals within a population at the same time than is expected based on recent experience. It's a sudden increase in the number of disease cases. If an epidemic spreads across multiple populations, sometimes even worldwide, it is referred to as a pandemic. An example of a pandemic disease is HIV/AIDS. Infectious diseases such as the flu or cold are examples of contagious illnesses that can lead to epidemics. Epidemiologists are the scientists who study disease patterns within populations to understand and combat these occurrences.