[Illustration: FIG. 64.—Plague bacillus involution forms produced by growing on 3% salt agar. (Kolle and Wasserman.)] =Epidemiology.=—Plague is primarily a disease of rodents, usually rats, and man contracts his infection from these animals. With the exception of pneumonic plague which, under certain circumstances, is transmitted directly from man to man, plague infections originate from the bite of fleas which have become infected from feeding on the blood of plague rats. Infected fleas act as intermediaries in plague epizootics among rats. It is true that a rat might become infected from bites received in a fight with an infected rat, or man might be infected through a cut on a finger while handling plague material, but such methods play but a small part in plague epidemiology. _Fleas and Plague._—In 1897, Ogata infected mice by inoculating them with an emulsion of crushed fleas taken from plague rats. In 1898 Simond showed that if a rat, dead of plague, were placed in a large bottle and a healthy rat confined in a small cage introduced into the bottle and suspended above the dead rat, so that there could be no contact between the dead and the living animal, the well rat would contract the disease. If however the fleas were removed from the dead rat, before the introduction of the caged rat, no infection took place. By reason of claims that the rat flea would not bite man these convincing experiments were in a measure disregarded. The complete confirmation of the correctness of this view, as to transmission of bubonic plague, was brought about by the Indian Plague Commission. In a large number of experiments it was shown that when healthy and plague-infected guinea pigs were confined together in spaces where there were no fleas, there were no plague infections of any of these well animals. [Illustration: FIG. 65.—1 and 2, male and female _Xenopsylla cheopis_. 3, Head of _Ceratophyllus_. 4 and 5, male and egg-distended female of _Sarcopsylla penetrans_. _Xenopsylla cheopis_ is the most important plague transmitter of the flea family as it is the common rat flea of India where there is so much plague. _Sarcopsylla_ or _Dermatophilus penetrans_ is an important factor in a disabling skin disease, especially of the feet, in many parts of the tropics.]