The importance of Carlos Finlay (1833–1915) in the discovery of the mosquito vector of yellow fever has often been overshadowed by the fame of his American colleague and friend, Walter Reed. A Cuban physician of Scottish ancestry, Finlay trained in France and at Jefferson Medical College in Philadelphia. In 1881, he published his theory that a microbial agent transmitted from person to person by mosquito bites was the cause of yellow fever. Isolated in his views for the rest of the century, Finlay performed careful mosquito breeding experiments and identified the correct yellow fever vector, Aedes aegypti (then called Stegomyia fasciata or Culex fasciatus). However, he failed to produce experimental yellow fever in human volunteers by exposing them to mosquitoes fed on yellow fever patients, possibly because he did not recognize the long incubation period of the yellow fever agent inside the mosquito.
Finlay was also one of several nineteenth-century bacteriologists who mistakenly believed that they had isolated a specific microbial cause of yellow fever (shown, in the 1930s by Max Theiler, to be a virus), but Finlay's tetragenus, like Guiseppe Sanarelli's Bacillus Icteroides and Domingo Freire's micrococcus, proved to be a harmless contaminant. The experimental proof of the mosquito transmission pathway was obtained in Walter Reed's experiments with U.S. Army volunteers near Havana in the summer of 1900. Finlay supplied the eggs from which the mosquitoes used in the experiments were hatched, and Reed was careful to credit the importance of Finlay's work in each of his publications describing his experiments. Finlay served as president of the Cuban Board of Health for several years in the early twentieth century.