Smallpox is a potentially deadly infection caused by the variola virus. The infection causes severe illness, including a high fever, followed by a rash all over the body that looks like blisters.
Smallpox symptoms develop about 12 days after exposure. The disease is not contagious until the rash first appears. As scabs form, the chance of passing on the disease decreases.
Smallpox was a worldwide problem in the past. But because of a widespread vaccination program, it was declared eradicated (no longer a naturally occurring disease) by the World Health Organization (WHO) in 1980.
There may be a risk that the smallpox virus could be manufactured by people in order to threaten or harm others.
WebMD Medical Reference from Healthwise