Smallpox is a disease caused by the Variola major virus. Some experts say that over the centuries it has killed more people than all other infectious diseases combined. Worldwide immunization stopped the spread of smallpox three decades ago. The last case was reported in 1977. Two research labs still house small amounts of the virus. Experts fear bioterrorists could use the virus to spread disease.

Smallpox spreads very easily from person to person. Symptoms are flu-like and include high fever, fatigue and headache and backache, followed by a rash with flat red sores.

The U.S. stopped routine smallpox vaccinations in 1972. Military and other high-risk groups continue to get the vaccine. The U.S. has increased its supply of the vaccine in recent years. The vaccine makes some people sick, so doctors save it for those at highest risk of disease.