Infection occurs from flea bites and can go from people to rats, people to people, or rats to rats.
The Center for Disease Control has an excellent site with information on plague , including diagnosis, treatement, and vaccine info. The Rat Flea, Xenopsylla cheopsis , in the order, Siphonaptera . A contagious, often fatal epidemic disease caused by the bacterium, Yersinia pestis , transmitted from person to person or by the bite of fleas from an infected host, especially a rat, and characterized by chills, fever, vomiting, diarrhea, and the formation of buboes.