4. The blood of a yellow fever patient in the first three days, which was sterile for _B. icteroides_, was capable of producing the disease when injected subcutaneously. If heated to 55°C. for ten minutes the virus was destroyed however. Furthermore, infectious blood when passed through a Berkefeld filter remained infectious, thus showing that the virus is a filterable virus. The French Commission verified these findings and in addition brought out the following points. 1. Cutaneous vaccination with infective serum is without result. 2. Infectious serum loses its yellow fever-producing power in forty-eight hours unless preserved under liquid petrolatum when it remains virulent for five days. 3. Infectious serum if heated for five minutes at 55°C. loses its virulence but has prophylactic and curative power. 4. Besides the method of hypodermic inoculation yellow fever can only be transmitted by the bite of a mosquito in which the virus has remained for at least twelve days. 5. In one instance it was thought that the progeny of infected mosquitoes transmitted the disease. Rosenau and Goldberger, in 38 experiments, failed to obtain such result. _Paraplasma flavigenum._—In 1909, Seidelin reported certain minute protozoa as existing in the red cells of yellow fever patients. He considered them as related to the piroplasms and gave the name _Paraplasma flavigenum_. It is stated that the parasite has been found as late as the fourteenth day from the onset of the attack. The idea is advanced that there may be carriers of yellow fever. These claims are generally denied. =Epidemiology.=—There are numerous records which attest the almost universal susceptibility to yellow fever. In the Orwood epidemic, Carter has reported that of 46 persons entering an infected house, “Gray Mansion,” 45 contracted yellow fever. On the “Lombardia,” with a complement of 249, there were 242 cases and 134 deaths. The 7, who escaped, were immunes. The idea that the colored race possesses immunity is now thought to be connected with the contraction of the disease in infancy or childhood, attacks at this period of life being very mild and difficult of diagnosis. _Immunity._—As proof that such immunity is not racial we may note that in Ecuador the natives of the endemic area about Guayaquil possess an immunity due to mild attacks in childhood, but the natives of Quito, 300 miles distant, where there is no yellow fever, do not possess it, and many residents of Quito have contracted the disease when passing through Guayaquil to take steamer to Europe.