MODEL RELEASED. Smallpox infection. Simulation of a doctor taking a pus sample from a sore on a person infected with smallpox. Smallpox is a contagious viral disease unique to humans. It is caused by the virus variants Variola major and Variola minor. The virus is spread by prolonged contact with infected bodily fluids or contaminated objects. The virus attacks skin cells, causing the development of pustules (blisters) all over the body. In the worst form of the disease, the pustules merge together and the skin separates from the underlying flesh. A global vaccination program led to the complete eradication of smallpox by 1979. However, stocks of the virus are held by the US and Russia.