Death from yellow fever
"I believe that I am on the trail of the true pathogen," wrote the American physician Jesse Lazear on September 8, 1900, in a letter to his wife. Lazear researched malaria and yellow fever. He confirmed that the latter is transmitted by mosquitoes. To study the disease, he intentionally allowed himself to be stung, fell ill and died 17 days after writing the letter. Lazear was only 34 years old.