4. Walter Reed
Walter Reed was a U.S. Army physician, who spearheaded research on the disease yellow fever. While working in Cuba, Reed believed that his patients who were contracting the disease, were doing so as a result of being bitten by the local mosquitoes, however, he had no physical proof that this was the case. Based upon how quickly the fever could spread throughout a community, Reed determined that a mosquito that bit an infected patient could spread it to another via a bite.
To test this theory, Reed experimented on himself along with three colleagues. All three contracted the fever and returned to the United States. Although Reed survived, his fellow test subjects died. Reed eventually returned to Cuba following the breakthrough, but never tested himself again. Sadly, he would die a year later at the age of 51.