Smallpox disease had a mortality rate of approximately 30 percent, and because of a successful vaccination program it was declared globally eradicated in 1980. Naturally occurring smallpox was highly contagious, and was transmitted from an infected person to a susceptible (unvaccinated) person by close contact with respiratory secretions and/or infected skin. When smallpox disease was circulating, the only known reservoir for the virus was humans.
Once infected, a person usually has no symptoms and is not contagious during the incubation period, which ranges from 7-17 days. Symptoms then begin with high fever, headache, body aches and fatigue. This may last 2-4 days, and then the rash develops. The rash first appears on the oral mucosa, face, and forearms, then spreads to the trunk and legs. A person is most infectious during the first week of the rash and is no longer infectious after all scabs have separated, usually 3-4 weeks after onset.
There is currently one smallpox vaccine licensed in the United States, ACAM2000, and is given in a pre-event setting only to select, designated groups including certain members of the U.S. military. The smallpox vaccine is administered in a single dose by the percutaneous route (scarification) using 15 jabs with a stainless steel bifurcated needle that has been dipped into the reconstituted vaccine. A few days following successful administration of the vaccine, a lesion forms at the vaccination site. This lesion contains vaccinia virus capable of infecting other parts of your body or infecting others who come in contact with the vaccination site directly or anything that contains drainage from the lesion.
Source: Centers for Disease Control and Prevention