Walter Reed led the commission that proved yellow fever is transmitted by mosquitoes.
In 1900, U.S. Army Surgeon General George Sternberg appointed Reed to head the Army Yellow Fever Commission in Cuba. The team included James Carroll, Jesse Lazear, and Aristides Agramonte. Their experiments showed that yellow fever was spread by the bite of a particular mosquito, now known as Aedes aegypti, rather than by direct contact or contaminated clothing and bedding.
The work was dangerous: researchers and volunteers allowed themselves to be exposed to infected mosquitoes, and Lazear died after contracting yellow fever during the investigations. The findings quickly influenced public-health campaigns, especially mosquito-control efforts in Cuba and Panama.
Carlos Finlay is a common alternative in this question because he had proposed the mosquito-transmission theory decades earlier and identified the likely mosquito. William Gorgas later applied mosquito-control measures on a large scale. Reed’s specific role was leading the commission that experimentally confirmed the mechanism.