The spirochætes occur in the body fluid of the lice and can pass in it to the adjacent organs. Thus they probably find their way into the genital organs, and into the eggs of the lice. Eggs laid twenty to thirty days after the parent became infected have retained the infection, and the larvæ issuing from such eggs must have contained some form of spirochætes, for an emulsion of either the eggs or the larvæ produced spirochætosis when inoculated into monkeys. Further details regarding the spirochætosis in the eggs of the lice and in the larvæ are needed. Hereditary infection, however, has been demonstrated, but is not very common. Sergent and Foley (1914) state that the spirochæte possesses a very small and virulent form which it assumes during apyrexial periods in man and during a period following an infecting meal in the louse. Nicolle and Blanc (1914) find that the organisms are infective in the louse just before they reappear as spirochætes. Nicolle and Blaizot found that female lice were more susceptible to spirochætes than males, four times as many females as males being infected. Tictin (1897) found _S. recurrentis_ in bugs recently fed on patients, and infected a monkey with the fluids of crushed bugs. Karlinski (1902) found the spirochætes in bed-bugs in infected houses. There is some other evidence to show that bugs may transmit the spirochæte in Nature. Further researches are needed regarding the relationship of bed-bugs and human spirochætosis. Multiplication of _S. recurrentis_ is by longitudinal and transverse division (including so-called “incurvation”), and the organism forms small, ovoid bodies (“coccoid” bodies) in the same way as _S. duttoni_. _S. recurrentis_ is the cause of European relapsing fever, and a number of possible varieties of it are associated with relapsing fevers in other parts of the world. Such spirochætes only differ by biological reactions, such as acquired immunity tests. They include:-- _S. rossii_, the agent of East African relapsing fever; _S. novyi_, the agent of North American relapsing fever; _S. carteri_, the agent of Indian relapsing fever; _S. berbera_, the agent of North African and Egyptian relapsing fever. OTHER HUMAN SPIROCHÆTES are:-- _S. schaudinni._ This organism, according to Prowazek, is the agent of ulcus tropicum. It varies in length from 10 µ to 20 µ. _S. aboriginalis_ has been found in cases of granuloma inguinale in British New Guinea and Western Australia. It also occurs in dogs, and may not be truly parasitic. _S. vincenti._ This spirochæte is 12 µ to 25 µ in length, tapers at both ends and has few coils. It has been associated with angina vincenti. It often occurs in company with fusiform bacilli.