Plague is an infectious disease that can cause serious illness in people living in rural areas of the western United States and parts of Africa, Asia, and South America. It is caused by the bacteria Yersinia pestis, and people usually get plague after being bitten by an infected flea or handling an infected animal. It is possible that Y. pestis could be released intentionally in a biological attack to sicken people. Public health authorities, including CDC, have prepared for the possible but unlikely event that Y. pestis could be used as a biological weapon.