Plague is a disease caused by a bacterium called Yersinia pestis . The disease is usually passed from infected rodents to humans through fleabites. Several worldwide outbreaks of plague have been recorded in history, and there have been nearly 400 cases of plague in the United States within the past 50 years. A bioterrorist might release plague bacteria into the air, causing the rare "pneumonic" type, which affects the lungs. Pneumonic plague can be spread from person-to-person by respiring (breathing) droplets of infectious saliva. Between one and six days after someone is exposed, he or she develops fever, cough and difficulty breathing, and sometimes show blood in the secretions. There are antibiotics to treat plague, but in order to be effective they must be started as soon as it is determined that someone has been exposed or immediately after symptoms begin. There is no vaccine available to protect against pneumonic plague. If any cases of plague were discovered, public health officials would notify the public and those believed to have been exposed would be informed where to go for treatment.