The eradication of smallpox, announced in 1980, was hailed as one of humankind's greatest accomplishments. Because smallpox was no longer a threat, the World Health Organization asked all nations to stop vaccinating civilians. The United States had stopped in 1972, and all other countries ceased by 1984. However, the fear that Variola might be used as a biological weapon led a number of nations, including Australia, Canada, Israel, the Soviet Union, and the United States, to continue vaccinating their military personnel. By 1990, though, most countries had even stopped vaccinating soldiers.

The long-range consequence of these events is that most of the world's population is now susceptible to smallpox. Even previously vaccinated individuals may not be protected since vaccineinduced immunity declines over time. Today, there are mounting concerns in many nations that bioterrorists or outlaw countries may obtain smallpox viruses and unleash...