Bubonic plague is among the more virulent infectious diseases. Its association with humans is an ancient one.
Plague primarily is a disease of rodents.
Under certain circumstances, however, plague can be introduced to humans, where it is devastating. Fleas that have fed on plague-infected rats or other rodent species become infected. Plague bacilli multiply in the gut of the flea, and, if humans are in close association with infected rats, the fleas may attempt to feed on them. During feeding, the plague bacilli are introduced into the victim. In humans, after the bacteria are introduced, the onset of the disease usually occurs in three to four days.