Variola virus (or variola major virus) is the virus that causes smallpox. The virus is one of the members of the poxvirus group (Family Poxviridae). The virus particle is brick shaped and contains a double strand of deoxyribonucleic acid. The variola virus is among the most dangerous of all the potential biological weapons. At the time of smallpox eradication approximately one third of patients died—usually within a period of two to three weeks following appearance of symptoms.
Variola virus infects only humans. The virus can be easily transmitted from person to person via the air. Inhalation of only a few virus particles is sufficient to establish an infection. Transmission of the virus is also possible if items such as contaminated linen are handled.
The origin of the variola virus in not clear. However, the similarity of the virus and cowpox virus has prompted the suggestion that the variola virus is a mutated version of the cowpox virus. The mutation allowed the virus to infect humans. If such a mutation did occur, then the adoption of farming activities by people, instead of the formally nomadic existence, would have been a selective pressure for a virus to adopt the capability to infect humans.
Vaccination to prevent infection with the variola virus is long established.
In the late 1990s, a resolution was passed at the World Health Assembly that the remaining stocks of variola virus be destroyed, to prevent the re-emergence of smallpox and the misuse of the virus as a biological weapon. At the time only two high-security laboratories were thought to contain variola virus stock (Centers for Disease Control and Prevention in Atlanta, Georgia, and the Russian State Centre for Research on Virology and Biotechnology, Koltsovo, Russia). However, this decision was postponed until 2002, and now the United States government has indicated its unwillingness to comply with the resolution for security issues related to potential bioterrorism. Destruction of the stocks of variola virus would deprive countries of the material needed to prepare vaccine in the event of the deliberate use of the virus as a biological weapon. This scenario has gained more credence in the past decade, as terrorist groups have demonstrated the resolve to use biological weapons, including smallpox. In addition, intelligence agencies in several Western European countries issued opinions that additional stocks of the variola virus exist in other than the previously authorized locations.
See also Viral genetics.