_Infection by Injection of Blood._—The subcutaneous injection of as little as 0.1 cc. of the blood of a yellow fever patient in the first three or four days of the disease, or the serum of this blood, which has passed through the pores of a Chamberland filter (F but not B according to the French Commission), will bring about the infection of a susceptible person after an incubation period of from one day and fifteen hours to twelve days and eighteen hours. In the natural method of transmission the mosquito, _Stegomyia calopus_, is the intermediary. In order that the female of this culicine species may transmit the disease it is necessary that she bite a yellow fever patient in the first three days of his illness, after which a period of approximately twelve days must elapse before the mosquito can transmit the disease. In such case the period of incubation varies from two days one hour to six days two hours. Carroll was bitten by a mosquito which had fed on a yellow fever patient twelve days previously, and four days later experienced a very severe attack, this fact being against the views of the French Commission that the disease shows a less severe form in those who may be bitten in the first period of the infectivity of the mosquito. A second case bitten five days later by the same mosquito that infected Doctor Carroll had a mild attack. Persons bitten by experimentally contaminated mosquitoes before an interval of twelve days had elapsed escaped infection. Prior to the investigations of the American Commission our views as to the incidence and spread of yellow fever were chaotic. Rush, in 1793, thought that the Philadelphia epidemic originated from “damaged coffee which putrefied on a wharf near Arch Street.” In 1883, Doctor Friere reported that yellow fever was caused by a coccus, _Cryptococcus xanthogenicus_ and claimed that he could confer immunity by vaccination with attenuated cultures. Carmona y Valle of Mexico and Carlos Finlay of Havana considered that the _Micrococcus tetragenus_ was the cause but Sternberg, investigating these claims, showed that these cocci had nothing to do with yellow fever. In his work Sternberg isolated an organism which he designated “X.” =Bacillus Icteroides.=—In 1897, Sanarelli isolated an organism which he named _Bacillus icteroides_. In investigating this organism Reed and Carroll found that it was closely related to the hog cholera bacillus. Certain American investigators substantiated the claims of Sanarelli. Sternberg, however, doubted these findings. To further investigate the relation of _B. icteroides_ to yellow fever Army surgeons were sent to Cuba in 1900. =American Commission.=—In addition to Reed and Carroll, Lazear and Agramonte were also members of the Commission. The Commission first cultured the blood of 18 yellow fever patients with negative results for _B. icteroides_ in every case.