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Spurred by the massive yellow fever-related casualties in the Spanish-American War, members of the U.S. Army Yellow Fever Commission, headed by Walter Reed, traveled to Cuba to study the disease. Commission member Jesse Lazear (1866-1900) met Henry Rose Carter, a surgeon who had studied the incubation period of yellow fever. Carter gave Lazear a draft of a paper he’d authored defining the incubation period for the disease as a range of 10 to 17 days. Lazear reportedly stated that Carter’s dates, should they prove to be correct, suggested a living host for yellow fever. Determined to uncover the source of the illness, the Commission members visited Carlos Finlay and decided to test his theory of mosquito transmission.

Jesse Lazear hatched Finlay’s mosquito eggs and let the mosquitoes feed on patients infected with yellow fever at a Havana hospital. The mosquitoes were then allowed to feed on study volunteers over a period of two weeks—yet no infections resulted. Two days later, however, Lazear once again allowed the mosquitoes to feed. This time, both of the men who were bitten fell ill. These experiments validated the theory Finlay had presented two decades earlier: mosquitoes (specifically, the Aedes aegypti variety) were the transmission vector of yellow fever. The researchers went on to rule out a bacterium as the disease agent. They determined that an infectious particle too small to be filtered with a standard bacterial filter was the source of the disease: the first human virus ever discovered. (German scientists had identified the virus that caused foot-and-mouth disease in animals in 1898.)

The two men Lazear exposed to yellow fever via the experiment’s mosquitoes recovered. Lazear himself, however, was not so lucky. It is likely that he allowed himself to be bitten as part of the experiment. Lazear contracted yellow fever and died in September 1900, at age 34.

The Army experiments, however, continued, with Reed naming newly designed facilities “Camp Lazear.” The group proceeded to determine that the mosquitoes could transmit the disease only after a certain period of time had passed since they had fed on another human infected with it (in the range of 12-20 days) and that a victim bitten by an infected mosquito would typically fall ill within six days.

William Crawford Gorgas (1854-1920), a Colonel in the U.S. Army Medical Corps, later described the details of the discovery:

They… found out that [a man], before he had been bitten by the yellow-fever mosquito, could sleep in the bed in which a patient had died of yellow fever, could be covered with a black vomit from a yellow-fever patient, or be exposed to the emanations from yellow fever in any other way, and as long as he was kept safe from the bite of the mosquito he would not have yellow fever; but this same man, after all this exposure, if afterwards bitten by an infected mosquito, would very certainly catch the disease.

…They had a little frame building built in this camp furnished with jars and the necessary simple material for breeding mosquitoes… Eggs of this particular species of mosquito were obtained and hatched in one of the jars. A female mosquito was taken from the booth thus hatched. The male mosquito will not bite… The female mosquito selected was put into a small glass tube, stoppered with a little cotton, so that she could get air but not escape, taken to Habana, placed on the hand of a patient in the first three days of an attack of yellow fever, and allowed to fill herself with blood. She was then brought back to her former home, placed in a large glass jar, and allowed to digest the blood she had obtained.

…So confident were the men in charge of the mosquitoes that I have known them to put their hands in the jars and let the mosquitoes feed upon them, up to the fifth or sixth day after the mosquito had bitten a yellow-fever patient. The mosquito, you recollect, can not convey the disease till from 12 to 20 days have passed from the time of her biting the yellow-fever case from which she becomes infected.

On the other hand, I once saw a party of 12 or 15 doctors in the mosquito room one day, when the mosquito-bar covering of the jar accidentally came off and the insects escaped into the room. These doctors had come from other countries to investigate the subject, and were not then convinced that the mosquito carried yellow fever. Still, they did not care to put the matter to a practical test in their own persons, and got out of the room so rapidly that the wire-screen door was broken down during their exit. It happened that the mosquitoes in this jar had never bitten a yellow-fever patient and were not infected.

-- Colonel William C. Gorgas, Medical Corps, United States Army, A few general directions with regard to destroying mosquitoes, particularly the yellow-fever mosquito

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