But all of the surmises and theories came short of the truth until Reed, Carroll, Lazear, and Agrimonte (Lazear at the cost of his life and Carroll at the cost of a nearly fatal attack of yellow fever) took up the work of proving that there was only one way in which yellow fever could be transmitted; namely, by the bite of the mosquito. Sleeping with patients who had yellow fever, wearing the clothes of those who had died from it, eating from utensils from which yellow fever victims had eaten--in short, putting to the most rigid test every other possible method of infection, they proved by every negative test that yellow fever could not be produced in any way other than by the bite of a mosquito. The next step was to give affirmative proof that yellow fever was caused by the bite of the female "stegomyia"--she of the striped stockings and the shrill song. This meant that someone had to have enough love for humanity to risk his life by inviting one of the worst forms of death to which human flesh is heir. Those doctors knew that they could not as brave men ask others to undergo the risks that they themselves might not accept, so in a little council chamber in Havana the three Americans--Reed, Carroll, and Lazear--entered into a compact that they themselves would permit infected mosquitoes to bite them. Reed was called home, but Carroll and Lazear stood with the keen and cold eyes of scientists and saw the mosquitoes inject the fateful poison into their blood. Later, after Lazear had died and Carroll had stood in the jaws of death, soldiers of the American army in Cuba volunteered in the interest of humanity to undergo these same risks. And it was thus, at this price, that the world came to know how yellow fever is caused, and that the United States was to be able to build the Panama Canal. After the guilt of the female "stegomyia" mosquito was firmly established the next problem was to find a method of combating her work. Dr. Reed and his associates thought that it might be done through a process of immunization, using the mosquito to bite patients with very mild cases, and after the necessary period of incubation, to transmit the disease to those who were to be rendered immune. It was soon found, however, that there was no method of transmitting a mild infection, and the next problem was to combat the work of the mosquito by isolation of yellow fever patients, and by the extermination of the mosquitoes themselves.