Hearing the word epidemic, one often thinks first of the flu, measles, the ACQUIRED IMMUNODEFICIENCY SYNDROME (AIDS), or some other contagious disease spreading through a community. In epidemics with person-to-person spread of infection and disease, people become infected and fall victim to the disease, and in the process they come into contact with other people, who in turn get the infection and disease. Often, what is being spread from person to person is not the disease itself, but rather an agent of the disease—for example, one of the viruses that accounts for influenza, the measles virus, or the human immunodeficiency virus (HIV) that causes AIDS.

In EPIDEMIOLOGY (the study of epidemics), it is not the agent, the person-to-person spread of a disease, or the intentional or unintentional nature of acquiring the infection or disease that defines an epidemic. Instead, an epidemic...