First experimental yellow fever mosquito bite

In March 1901, Clara Maass volunteered as a subject in yellow fever experiments conducted under the auspices of the U.S. Army's Yellow Fever Board, building on the earlier work of Walter Reed and the Cuban physician Carlos Finlay, who had hypothesized that mosquitoes — not direct contact or fomites — transmitted yellow fever.

Maass, then a contract nurse stationed in Cuba, allowed herself to be bitten by a mosquito that had previously fed on a yellow fever patient. This first exposure was intended to test whether deliberate, controlled infection could produce immunity in volunteers, a method researchers hoped might eventually lead to a safe form of inoculation.

Her willingness to participate reflected both the scientific fervor of the era, when human experimentation ethics were far less regulated than today, and the immense personal courage required given the high mortality rate of yellow fever. Maass reportedly experienced only a mild case of the disease after this first bite, which encouraged further experimentation.

This event marked the beginning of a series of exposures that would ultimately cost Maass her life, but which contributed critical data confirming the mosquito-transmission theory — a discovery that revolutionized public health strategy and led to mosquito-control campaigns that dramatically reduced yellow fever mortality throughout the Americas.