Pestis: The plague. An infectious disease due to a bacteria called Yersinia pestis. Y. pestis mainly infects rats and other rodents. Rodents are the prime reservoir for the bacteria. Fleas function as the prime vectors carrying the bacteria from one species to another. The fleas bite the rodents infected with Y. pestis and then they bite people and... Found on http://www.medterms.com/script/main/art.asp?articlekey=8547