A peculiar feature of the Rocky Mountain spotted fever is a marked variation in its severity in different localities. In Montana, and especially in the famous Bitter Root Valley, from 33 per cent to 75 per cent of the cases result fatally. On the other hand, the fatality does not exceed four per cent in Idaho. In 1902, Wilson and Chowning reported the causative organism of spotted fever to be a blood parasite akin to the _Babesia_ of Texas fever, and made the suggestion that the disease was tick-borne. The careful studies of Stiles (1905) failed to confirm the supposed discovery of the organism, and the disease is now generally classed as due to an invisible virus. On the other hand, the accumulated evidence has fully substantiated the hypothesis that it is tick-borne. According to Ricketts (1907) the experimental evidence in support of this hypothesis was first afforded by Dr. L. P. McCalla and Dr. H. A. Brereton, in 1905. These investigators transmitted the disease from man to man in two experiments. "The tick was obtained 'from the chest of a man very ill with spotted fever' and 'applied to the arm of a man who had been in the hospital for two months and a half, and had lost both feet from gangrene due to freezing.' On the eighth day the patient became very ill and passed through a mild course of spotted fever, leaving a characteristic eruption. The experiment was repeated by placing the tick on a woman's leg and she likewise was infected with spotted fever." The most detailed studies were those of the late Dr. H. T. Ricketts, and it was he who clearly established the tick hypothesis. In the summer of 1906 he found that guinea pigs and monkeys are very susceptible to spotted fever and can readily be infected by inoculation of blood from patients suffering from the disease. This opened the way to experimental work on tick transmission. A female tick was fed upon an infected guinea pig for two days, removed and isolated for two days and then placed upon a healthy guinea pig. After an incubation period of three and a half days the experimental animal contracted a well-marked case of the disease. A similar result was obtained at the same time by King, and later in the season Ricketts proved that the male tick was also capable of transmitting the disease. He found that there was a very intimate relation of the virus to the tick and that the transmission must be regarded as biological throughout. Ticks remained infective as long as they lived and would feed for a period of several months. If they acquired the disease in the larval or nymphal stage they retained it during molting and were infective in the subsequent stages. In a few cases the larvæ from an infected female were infective.