The experiments conducted at Camp Lazear in Cuba in 1900, led by U.S. Army Major Walter Reed, aimed to definitively determine the mechanism of yellow fever transmission. The prevailing theory held that the disease was spread by fomites—contaminated objects like clothing and bedding. The Yellow Fever Board sought to scientifically disprove this belief while testing the alternative hypothesis that a mosquito was the vector. The rigorous design of the camp created two completely isolated and controlled testing environments, ensuring any outcome could be attributed solely to the variable being tested.
Establishing the Research Environment
Camp Lazear was established in an isolated area near Quemados, Cuba, on the outskirts of an American military base. This remote location was chosen to prevent uncontrolled external exposure from compromising the results, allowing researchers to maintain total control over the non-immune volunteers and the environment.
Volunteers were primarily non-immune U.S. Army personnel and recent Spanish immigrants. They were offered a financial incentive: $100 for participation and an additional $100 if they contracted yellow fever. Volunteers were presented with written contracts outlining the study’s risks, a foundational step toward modern informed consent protocols.
The commission set up seven tents on wooden floors for the volunteers’ housing outside of the experimental structures. This careful isolation and the use of signed, translated contracts secured the scientific integrity and ethical defensibility of the experiments. The entire setup was designed to eliminate alternative sources of infection, ensuring the experimental buildings were the only possible origin of the disease.
The Fomites Experiment Setup
The first specialized structure, the “Infected Clothing House” (Building No. 1), was designed to test the theory that yellow fever was transmitted by contact with contaminated materials. The small, wooden building had extremely poor ventilation, with only two small windows, and was kept deliberately hot and humid using a stove. This created the ideal tropical conditions thought to foster the disease.
The experimental protocol required volunteers to sleep in the house for up to 20 consecutive nights. Bedding, clothing, and towels used by yellow fever patients were deliberately soiled with bodily discharges like blood and black vomit and spread throughout the room.
Volunteers were instructed to unpack these items, shake them out to simulate the spread of contagious particles, and then sleep on the soiled mattresses and linens. This setup exposed the men to the fomite theory’s conditions in their most extreme form. Crucially, the house was kept strictly free of mosquitoes, ensuring contamination from the objects was the only variable tested.
The Mosquito Transmission Setup
The second structure, the “Infected Mosquito House” (Building No. 2), was the opposite of the fomites house, isolating the mosquito as the sole variable of infection. This building was clean, well-ventilated, and protected by window screens to ensure a sterile environment. A fine-mesh wire screen partition divided the structure into two distinct sections, allowing air circulation but preventing mosquitoes from passing between the sides.
The experiment relied on the precise control of the Aedes aegypti mosquito, the species implicated by Carlos Finlay’s work. Mosquitoes were allowed to feed on confirmed yellow fever patients during the first three days of illness, when the virus was circulating in the blood. The infected female mosquitoes were then held for a minimum of 12 days, the period necessary for the virus to incubate and become infectious within the insect.
Volunteers were placed on the side of the partition where the infected mosquitoes were confined. They exposed specific parts of their bodies to be bitten by the carefully timed insects. Other volunteers slept in the second, clean compartment, which was free of infected insects, serving as an environmental control to show the house itself was not the source of infection.
Monitoring and Documentation Protocols
The Yellow Fever Board implemented rigorous scientific methods for tracking and documenting the results of every exposure. Once exposed to either the fomites or the infected mosquitoes, volunteers were subject to constant medical observation, including the immediate isolation of anyone showing the earliest symptoms of illness.
The medical team maintained detailed clinical records, using temperature charts taken at frequent intervals to identify the onset of fever. This close monitoring established a precise timeline from exposure to the first appearance of symptoms, helping determine the disease’s incubation period.
Control groups were integral to the design, including individuals living in sterile tents who were not subjected to either experimental house, and those who slept in the clean side of the Infected Mosquito House. The comprehensive documentation and use of controls allowed the team to conclude that only volunteers exposed to the infected Aedes aegypti mosquitoes contracted yellow fever, providing irrefutable proof of the vector.