Smallpox
Smallpox, a disease which is caused by the variola virus, manifests itself in several forms. The most common form, ordinary smallpox, is a characterized by a high fever and an extensive rash.

The incubation period for smallpox averages 12 to 14 days, and during this period people infected with the disease feel healthy and are not contagious. The initial symptoms of smallpox include a fever ranging from 101 to 104 degrees. After two to four days, a rash emerges first as small red spots on the tongue and mouth. At this time people with the disease are at their most contagious. Within 24 hours, a rash appears on the skin. This rash progresses for about two weeks, at which point the spots have scabbed over. Once the scabs have fallen off, a person is no longer contagious.

Direct and fairly prolonged face-to-face contact with a person who has the smallpox rash is the only way to pass the disease from one person to another. In a terrorist attack, exposure to the disease could also come from breathing in the airborne virus. Humans are the only natural hosts of smallpox -- animals do not carry or spread the disease.

A vaccine for smallpox has been developed, but there is no specific treatment for the disease. Historically, ordinary smallpox has a 30 percent fatality rate.