The outcome of a variola virus infection will depend on the type of smallpox a person develops. For variola major, the overall fatality rate is about 30 percent. Flat and hemorrhagic smallpox are usually fatal. Variola minor is a less common presentation of smallpox, and a much less severe disease, with death rates historically of 1 percent or less.
In the aftermath of the events of September and October 2001, there is heightened concern that the variola virus might be used as an agent of bioterrorism. The deliberate release of the virus is now regarded as a possibility, and the United States is taking precautions to deal with this threat.
The Centers for Disease Control and Prevention (CDC) considers the variola virus a Category A agent. Category A agents are believed to present the greatest potential threat for harming public health and have a moderate to high potential for large-scale dissemination. The public is generally more aware of Category A agents, and broad-based public health preparedness efforts are necessary. Other Category A agents are:
Scientists have not studied variola virus well because of the hazards associated with potential exposure. In addition, by international agreement, smallpox may only be studied at the CDC high-containment facility or in the former USSR (Union of Soviet Socialist Republics), and experiments must be approved in advance by an international committee.
Both vaccinia virus and variola virus are from the same family. The vaccinia virus is used to make a smallpox vaccine and has been studied thoroughly.
There is one major difference between the two viruses: Vaccinia virus can infect several types of living beings, while variola virus infects only humans naturally and cynomolgus monkeys under highly artificial laboratory conditions.