The immunity conferred by an attack is not lasting and a second infection may occur within a year. Such second attacks, however, do not present the relapses so important in a clinical diagnosis. As a matter of fact there may be no symptoms and such cases with spirochaetes in their blood make ideal carriers for the infection of ticks or lice. Ticks can be infected by these carriers. In some of the rest houses 50% of the ticks may be infected. While the tick does not tend to leave its habitation it may be transported in the bundles of native porters. The transmitting agent of the North African relapsing fever and probably of the Indian type is the louse. The body louse deposits about 75 eggs in the clothes of the host, which hatch out in about four days and become adults in about two weeks. The head louse deposits its eggs or nits on the hair of the host’s head. Hagler has noted that, in Servia, typhus fever disappeared when lice were gotten rid of but relapsing fever continued to prevail until they also exterminated the bedbugs. _The Relapsing Fever of Panama._—Bates, Dunn and St. John have demonstrated that a tick, _Ornithodoros talaje_, transmits the relapsing fever of Panama. This tick seems to prefer the crevices and open joints of bamboo beds, laying its eggs and moulting in the hiding places. It comes out at night to feed on the occupants of the bed. Two white rats were inoculated with an emulsion made from ticks taken from a suspected bed. Both rats developed a spirochaete infection. A man inoculated with blood of one of these rats developed relapsing fever on the sixth day. A second man was inoculated subcutaneously with an emulsion of ticks collected from a bed and showed spirochaetes in his blood on the eleventh day. Another man was bitten by ticks from the same source and was instructed not to scratch the bite. The ticks engorged in about 15 minutes, some of them secreting coxal fluid which mixed with blood from the bite made a scab. This man was positive for spirochaetes on the fifteenth day. PATHOLOGY AND MORBID ANATOMY The spirochaetes disappear from the peripheral circulation during the apyrexial period, notwithstanding which such spirochaete-free blood, when injected into monkeys, may bring about infection. Either a granule stage or an invisible stage of the parasite may be present. The relapse is probably due to the existence of resistant strains which are not destroyed by the lytic substances, developed during the attack. Agglutinating and lytic substances show themselves chiefly during the apyretic intervals. The spleen is enlarged and soft. There are frequent infarctions. The spirochaetes are found phagocytized in the macrophages of the spleen and elsewhere. Parenchymatous degeneration of kidney and heart muscle, and especially of liver, may be noted. SYMPTOMATOLOGY