The plague is caused by the gram-negative bacillus Yersinia pestis. It is a zoonotic infection of rodents that can be transmitted through flea bites, but also person-to-person. The plague is more difficult to use as a biological weapon than anthrax, because Y. pestis does not form spores, it is susceptible to drying, heat, and ultraviolet light, and does not survive well outside the host body. Therefore, so far there has not been an effective bioweapon using aerosolized bacteria [38] . Unlike anthrax, secondary cases may result from person-to-person transmission [10] , however this requires close contact with a patient during the final stage of the illness [39] . Experts believe that the danger of terrorists using this organism may be greatly exaggerated [40] .