An epidemic is an outbreak of a disease that involves a large number of people in a contained area (e.g., village, city, country). An epidemic that is worldwide in scope is referred to as a pandemic. A number of viruses have been responsible for epidemics. Some of these have been present since antiquity, while others have emerged only recently.

Smallpox is an example of an ancient viral epidemic. Outbreaks of smallpox were described in 1122 B.C. in China. In A.D. 165, Roman Legionnaires returning from military conquests in Asia and Africa spread the virus to Europe. One third of Europe's population died of smallpox during the 15-year epidemic. Smallpox remained a scourge until the late eighteenth century. Then, Edward Jenner devised a vaccine for the smallpox virus, based on the use of infected material from cowpox lesions. Less than a century later, naturally occurring smallpox...