 From 1901 to 1905 he participated on a mission to study yellow fever in Brazil where he and his colleagues confirmed the results that the U.S. Army Commission led by Walter Reed had just obtained in Cuba.These revealed that the amaril virus is the pathogen present in the blood of patients, and that this virus can be conveyed to humans by the mosquito species, "Stegomyia fasciata" (later renamed "Aedes aegypti").In 1908-09 he conducted yellow fever research in Martinique.