INTRODUCTION

Smallpox is caused by variola viruses, which are large, enveloped, single-stranded DNA viruses of the Poxvirus family and the Orthopoxvirus genus. Variola major strains cause three forms of disease (ordinary, flat type, and hemorrhagic), whereas variola minor strains cause a less severe form of smallpox. Vaccination with vaccinia virus, another member of the Orthopoxvirus genus, protects humans against smallpox because of the high antibody cross-neutralization between orthopoxviruses.

The Working Group for Civilian Biodefense considers smallpox a dangerous potential biological weapon because of “its case-fatality-rate of 30% or more among unvaccinated persons and the absence of specific therapy.” Of the potential ways in which smallpox could be used as a biological weapon, an aerosol release is expected to have the most severe medical and public health outcomes because of the virus's stability in aerosol form, low infectious dose, and high rate of secondary transmission. A single case of smallpox would be a public health emergency.

EPIDEMIOLOGY

Smallpox as a Biological Weapon

Smallpox has been used as a biological weapon in the distant past and has recently been a focus of bioweapons research. In the 18th century, British troops in North America gave smallpox-infected blankets to their enemies, who went on to suffer severe outbreaks. Defecting Russian scientists describe covert Russian operations during the 1970s and 1980s that focused on the development of more virulent smallpox strains and of missiles and bombs that could release smallpox.