At least four species of fleas (including Pulex irritans) which infest the common rat are known to bite man, and are believed to be the active agents in the transmission of plague from rats to human beings.
The related species of the Oestridae family, which include the widely disseminated chigoe or bicho do pe (Pulex penetrans), and the equally troublesome berne (Cutiterebra noxialis), which is so injurious to animals, are equally numerous.
Healthy rats contracted plague from infected rats when the only apparent means of communication between the two was the rat flea (pulex cheopis).
