What causes smallpox?
Smallpox is caused by variola virus. The incubation period is about 12 days (range: 7 to 17 days) following exposure. Initial symptoms include high fever, fatigue, and head and backaches. A characteristic rash, most prominent on the face, arms, and legs, follows in 2-3 days. The rash starts with flat red lesions that evolve at the same rate. Lesions become pus-filled and begin to crust early in the second week. Scabs develop and then separate and fall off after about 3-4 weeks. The majority of patients with smallpox recover, but death occurs in up to 30% of cases.
Anyone exposed to the smallpox virus may get smallpox. Even people who have been vaccinated for smallpox might become ill, because the duration of protection given by the smallpox vaccine is not fully understood. The smallpox virus can be easily spread from one person to another after coming into close (within 6 feet) contact with a person who has smallpox. The virus is often contained in the saliva droplets of a person with smallpox.