Isaac Baniyo stumbled through his final exam in English last November as a pounding headache and chest pain made it difficult for him to focus. Within days, his fever soared, and he was hacking up bloody mucus. The dead rats in his village should have been a warning sign: Baniyo had caught pneumonic plague, the contagious form of the disease responsible for history's most notorious epidemics, including the Black Death of the 14th century.

Baniyo lives in a small village in Uganda, but his case is of keen interest in other nations. In an era of cheap air travel and international terrorism, scientists and national security experts are concerned that, much like anthrax, plague could pose a threat as a biological weapon.

"It's been on national lists as one of the more serious biological agents for a long time," said Tom Ingelsby, director of the Center for Biosecurity of the University of Pittsburgh Medical Center. Plague bacteria lack the hard spores that makes anthrax so durable, but because it spreads so easily and kills more than one-third of those it infects in developing countries, the Centers for Disease Control and Prevention considers it a Category A bioterrorism agent, alongside smallpox and Ebola.