In the last century typhus was distinguished definitely and clearly from "typhoid" or "enteric" fever, and from "relapsing" or "famine" fever, with which it had previously been confounded. The bacterial germs causing enteric and relapsing fevers are now known, and have been isolated and cultivated, and the mode in which they are conveyed into the body of a previously healthy patient is ascertained. But until the past year we knew neither the parasitic germ which causes typhus fever nor the mode by which it passes from one individual to another. A vague idea that it was spread through the air prevailed. Typhus is remarkable for the frequency with which the nurses and doctors attending a case become infected. About 20 per cent. of those attacked by it die, but in persons above forty-five years of age the mortality is much greater--about half succumb. Dr. Nicole and his colleagues of the Institut Pasteur in Tunis have recently had the opportunity of studying typhus there. They found that the ordinary local monkey could not be made to take the disease. But a drop of blood of a typhus patient injected into a chimpanzee (which is far nearer akin to man) produced the disease after an incubation period of three weeks. This fact was definitely established. From what is now known as to relapsing fever, malaria, yellow fever, plague, and sleeping-sickness, it seemed probable that some migratory insect must be the carrier of the typhus infection from man to man. The typhus patients brought into the hospital at Tunis were carefully washed before admission, and no infection of other patients or nurses took place in the wards, although the cases were not isolated, and bugs were abundant. The only cases of infection which occurred were in persons who had the duty of collecting and disinfecting the clothing of the patients when admitted. This seems to exclude the bug as a carrier. The flea is excluded by the fact that in the phosphate mines of Tunis the flea is abundant, and bites both natives and Europeans. Yet when typhus fever broke out among the miners--although all were equally bitten by the fleas--no European was infected. The indication, therefore, was that if any insect is the carrier, it is neither the flea nor the bug, but probably the clothes-louse. Although the smaller monkeys cannot be directly infected with typhus fever from man, it was found that (as with some other infections) the bonnet monkey was susceptible to the infection after it had passed through the chimpanzee. Experiments were, therefore, made with clothes lice taken from a healthy man, and kept for eight hours without food. They were placed on a bonnet monkey which was in full typhus eruption. A day afterwards they were removed to healthy bonnet monkeys with the result that the healthy bonnet monkeys developed typhus fever. There is thus no doubt whatever that typhus fever can be carried in this way from bonnet monkey to bonnet monkey. The whole history of typhus fever