Smallpox: A highly contagious and frequently fatal viral disease that is characterized by a biphasic fever and a distinctive skin rash that leaves pock marks in its wake. Because of its high case-fatality rates and transmissibility and because people haven't been vaccinated against it in years, smallpox now represents a serious bioterrorist threat. The disease is caused by the variola virus. The incubation period is about 12 days (range: 7'17 days) following exposure. Initial symptoms include high fever, fatigue, headaches, and backaches. A characteristic rash, most prominent on the face, arms, and legs, follows in 2 to 3 days. The rash starts with flat red lesions that evolve in 2 to 3 days. Lesions become pus-filled and begin to crust early in the second week. Scabs develop and then separate and fall off after about 3 to 4 weeks. The majority of patients with smallpox recover, but death occurs in up to 30 percent of cases. Smallpox is spread from one person to another via infected saliva droplets as occurs during face to face contact. Persons with smallpox are most infectious during the first week of illness because that is when the largest amount of virus is present in saliva. However, some risk of transmission lasts until all scabs have fallen off. Also known as variola.