On October 23, 1900, U.S. Army Major Walter Reed presented the theory that mosquitoes were responsible for the transmission of yellow fever to the American Public Health Association in Indianapolis. His assertion followed months of tests by a four member U.S. Army Yellow Fever Board in Cuba. Only one month before, one of the board members who conducted many of the tests involving infected mosquitoes, Jesse Lazear, died from yellow fever.
Determined to prove the mosquito theory, Reed returned to Cuba, to a small quarantined area on the outskirts of an American military base west of Havana. In honor of his late colleague, he dubbed the testing area, Camp Lazear.
In the late 19th century, the crucial question with regard to yellow fever was, "How is the disease spread?" Observations from the numerous outbreaks in American and Cuba supported the idea that unsanitary conditions fostered the disease. Many believed that direct contact with an infected person, as well as contact with any infected clothing or bedding, could result in infection.
Carlos Finlay, a Cuban doctor and researcher, theorized that mosquitoes were responsible for transmitting the disease by biting an infected person and carrying the disease to an uninfected person. But he was unable to prove this theory with scientific certainty.
Walter Reed's team tested Finlay's theory in the summer of 1900 and uncovered a critical detail: test subjects who contracted yellow fever were bitten by mosquitoes that had been infected at least 12 days earlier. Reed surmised that the virus incubated within the mosquitoes before it became an infectious agent. Reed devised the Camp Lazear experiment to account for an incubation period.
At Camp Lazear, a building was set up to provide optimum conditions for infection by each means of transmission. Seven tents for volunteers were placed on the site, guarded by military personnel.
The "infected clothing building," as the team called it, was set up with three beds and a stove, so that a warmer temperature could be replicated. Yellow fever outbreaks were consistent with warm, summer weather. A collection of bedding, clothing and towels soiled with blood and other excretions from infected yellow fever patients were spread about the room and placed on the beds.
The other building, dubbed the "mosquito building," was divided into two sections, separated by a wire screen partition. A mosquito breeding room was connected to the larger of the two sections, which also had one clean, sterile bed. Two beds were set up in the smaller section, which was isolated from the mosquito breeding room. As with the other building, the interior was kept warm. An open vessel of water was set up in the larger room to provide a breeding ground for mosquitoes.
Reed and his team assembled a group of volunteers consisting of Army personnel and Spanish immigrants. Each volunteer was paid $100 to participate and received an additional $100 if infected.
Reed and his team first employed the infected clothing building on November 30, 1900. Three American volunteers entered the building to spend the night. They unpacked additional soiled clothes and began shaking them to spread any possible fomites. The stench from the items, soiled with bloody discharges, was so strong that one of the men had to run outside to vomit. For the next twenty nights, the three men slept in the building amidst the filthy items. During the day, they stayed together in one of the nearby tents.
Tests in the mosquito building commenced on December 21, 1900. Fifteen mosquitoes that had fed blood from yellow fever victims, were released into the larger compartment of the building. An American volunteer, John Moran, entered the larger compartment and lay down on the bed, dressed in sleeping garments. Three other Americans, including Walter Reed, entered the smaller compartment, separated from Moran by the screen partition. Over the next two days, during three sessions, Moran was bitten by numerous mosquitoes. The large compartment was then locked, but the three men in the smaller compartment continued to sleep in that space for nearly three weeks.
Volunteers in the infected clothing building suffered from the rank of the soiled materials but did not come down with yellow fever. John Moran, the first volunteer to be bitten by mosquitoes, did contract yellow fever, first showing symptoms on Christmas Day 1900. Five out of seven other volunteers in the larger compartment of the mosquito building also contracted yellow fever. Two of the five volunteers infected had also gone through the test in the infected clothing building, with no ill effects.
None of the men who slept in the smaller compartment contracted the disease.
In creating two isolated environments, Reed and his team were able to introduce volunteers into controlled experiments. There was absolute certainty in the results. They also used a multitude of volunteers, in case any of them had an immunity to yellow fever, which develops if one has already had the disease.
The only instances of contracting yellow fever occurred with people who were isolated in a sterile environment and exposed to mosquitoes known to have been infected with yellow fever with a proper incubation time. This last detail was the missing piece from Carlos Finlay's experiments.
Acknowledging the breakthroughs that the Cuban researcher had achieved, Reed welcomed Finlay to Camp Lazear to observe the tests.