This post is the fifth in an ongoing series about the history of the U.S. Army Yellow Fever Commission and the historical context of its work. The posts are and will become pages of a revamped online exhibit about the Commission. Some of the posts will contain content that has been previously published by the Claude Moore Health Sciences Library, while others will contain new content. The post preceding this one in the series is titled, The Havana Commission.
Carlos J. Finlay, a prominent Cuban physician who had served on Spain’s 1879 Havana Yellow Fever Commission, identified the link between yellow fever and the Aedes Aegypti mosquito long before the idea would become widely accepted in the world scientific community.
Finlay first recognized that mosquitoes may be the vector for yellow fever after studying the work of George Miller Sternberg. While serving on the U.S. Havana Yellow Fever Commission, Sternberg employed new technologies to capture microscopic images of blood extracted from yellow fever patients as they experienced the different stages of the disease. Finlay inferred from the Sternberg images, his own research about mosquitoes, and a wealth of existing epidemiological data that three conditions were necessary for the spread of yellow fever:
1. The existence of a yellow fever patient into whose capillaries the mosquito is able to drive its sting and to impregnate it with the virulent particles, at an appropriate stage of the disease.
2. That the life of the mosquito be spared after its bite upon the patient until it has a chance of biting the person in whom the disease is to be reproduced.
3. The coincidence that some of the persons whom the same mosquito happens to bite thereafter shall be susceptible of contracting the disease. 
When Finlay presented his mosquito theory at an 1881 meeting of the Academia de Ciencias Médicas, Físicas y Naturales de la Habana, the global scientific community generally dismissed it. Few other scientists of the period were proposing that mosquitoes could spread disease and, as Finlay himself acknowledged in his presentation:
I understand too well that nothing less than an absolutely incontrovertible demonstration will be required before the generality of my colleagues accept a theory so entirely at variance with the ideas which have until now prevailed about yellow fever. 
From 1881 to 1900, Finlay pursued a campaign of experimental inoculations on human volunteers, with the aim of demonstrating both the truth of his hypothesis and the possibility of inducing immunity to the disease. Finlay believed that he had produced cases of yellow fever by mosquito inoculation, although the larger public health community remained skeptical.  George Miller Sternberg offered an essential critique of Finlay’s experiments: that the participants were never sufficiently isolated from the general population to eliminate the possibility of contracting fevers from sources other than Finlay’s mosquitoes.  This and the inconsistency with which fevers developed in the experimental participants kept the mosquito theory on the margins of medical research until political developments on the international stage would compel others to take it up.
 Finlay, Carlos J. “The Mosquito Hypothetically Considered as the Agent of Transmission of Yellow Fever.” Translated by Carlos J. Finlay. Trabajos Selectos Del Dr. Carlos J. Finlay: Selected Papers of Dr. Carlos J. Finlay. Habana, Cuba, 1912. pp 39-41. Reprint of an article by Carlos J. Finlay that was first published in: Anales de la Academia de Ciencias Médicas, Físicas y Naturales de la Habana,Volume 18, 1881.
 For quote see: Ibid., pg. 42. In 1878, another relatively unknown Scottish physician, Patrick Manson, recognized a link between mosquitoes and the transmission of Elephantiasis. He soon after formulated a hypothesis that mosquitoes were the vector for malaria. The malaria-mosquito hypothesis would not become widely accepted until Ronald Ross carried out experiments in 1898 that proved Manson correct.
 Finlay, Carlos J. “Yellow Fever Its Transmission by Means of the Culex Mosquito.” The American Journal of the Medical Sciences. October, 1886. pp. 395-409. Culex Mosquito is an alternative name for Aedes aegypti.
 George Miller Sternberg, “Dr. Finlay’s Mosquito Inoculations,” American Journal of the Medical Sciences CII (1891) 6. ; Sternberg, George Miller. “The Transmission of Yellow Fever by Mosquitoes,”The Popular Science Monthly LIX (July, 1901) 3: 226-228.