At a meeting of the Havana Academy of Sciences, Carlos Finlay proposed that the Aedes aegypti mosquito, not direct contact or 'bad air,' spread yellow fever between people. The medical establishment largely ignored him. It took until 1900, when Walter Reed's U.S. Army commission confirmed Finlay's mosquito theory experimentally, for the idea to gain acceptance, leading to sanitation campaigns that dramatically curbed the disease in Cuba and Panama.
Carlos Finlay had spent years watching patterns in yellow fever outbreaks that didn't fit the prevailing theories of contagion. He noticed the disease seemed to follow water sources and warm seasons rather than direct human contact, and he suspected an insect vector. On this day he laid out his hypothesis formally, naming the mosquito as the culprit.
The scientific community's reaction ranged from skepticism to outright mockery. Finlay's experiments, in which he tried to infect volunteers by exposing them to mosquitoes that had bitten yellow fever patients, produced inconsistent results because he didn't yet understand the extrinsic incubation period the virus needed inside the mosquito before it became infectious.
Vindication came nearly twenty years later. Walter Reed's team in Havana, using Finlay's own mosquito-collection techniques, proved the theory beyond doubt. William Gorgas then used the finding to launch mosquito-eradication campaigns that made construction of the Panama Canal possible, saving thousands of workers who otherwise would have died of yellow fever. Finlay, once dismissed as a crank, is now honored across the Americas as the doctor who solved one of the deadliest riddles in tropical medicine.