Smallpox (variola) was a devastating disease with a high case-fatality rate. Although the disease was eradicated in 1977, the remaining stocks of smallpox virus constitute one of the most dangerous threats to humanity. The smallpox virus is highly specific for humans and non-pathogenic in animals. There is no antiviral treatment and a vaccine is active only if administered in the first four days post-exposure. Smallpox virus represents a potential biological weapon that could be used by terrorists, and the destruction of stocks raises political, social, scientific and ethical issues.