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 good information on plague . 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.