If the flea chances to be feeding on a plague-infected rat or person many of the plague bacilli will get on the mouth-parts and myriads of them are of course sucked up into the stomach with the blood. Those on the proboscis may be transferred directly to the next victim that it is thrust into, and those in the stomach may be carried for some time and finally liberated when the flea is feeding again or when it is crushed by the annoyed host. The latter is probably the most common method of infection, for the bacilli that are liberated when the flea is crushed may readily be rubbed into the wound made by the flea bite or into abrasions of the skin due to the scratching. Kill the flea, but don't "rub it in." [Illustration: FIG. 105--Rat-flea (_Læmopsylla cheopis_); male.] [Illustration: FIG. 106--Rat-flea (_L. cheopis_); female.] [Illustration: FIG. 107--Head of rat-flea showing mouth-parts.] [Illustration: FIG. 108--Human-flea (_Pulex irritans_); male.] During the recent outbreak in San Francisco many thousand fleas that were infesting man, rats, mice, cats, and dogs, squirrels and other animals have been studied and it has been found that while each flea species has its particular host upon which it is principally found, few if any of them will hesitate to leave this host when it is dead and attack man or any other animal that may be convenient. COMMON SPECIES OF FLEAS Throughout India and in all the warm climates where plague frequently occurs the most common flea found on rats has come to be known as the plague flea (_Læmopsylla cheopus_) (Figs. 105, 106), and is doubtless the principal species that is concerned in carrying the disease in those climates. It now occurs quite commonly on the rats in the San Francisco Bay region and is occasionally found there on man also. In the United States, Great Britain and other temperate regions another larger species, _Ceratophyllus fasciatus_ is by far the most common flea found on rats, and is commonly known as the rat flea. It occurs on both the brown and the black rats _Mus norvegicus_ and _M. rattus_, on the house mouse and frequently on man. It has also been taken in California on pocket gophers and on a skunk. The common human flea (_Pulex irritans_) (Figs. 108, 109), is found in all parts of the inhabited world. Although we regard it primarily as a pest of human beings it often occurs very abundantly on cats, dogs, mice and rats as well as on some wild mammals such as badgers, foxes and others and has occasionally been found on birds.