But since the days of the Lesseps company, science had thrown a flood of light on the nature of these tropical scourges and the secret of their transmission. As these medical and scientific pioneers made a Panama Canal possible, though their names are not directly linked with its construction, we may look back for a few moments at their triumphs of discovery. The credit for first discovering that malaria is not due to poisonous emanations or contagion but is carried from people infected with the disease by the _anopheles_ mosquito belongs to Major (now Sir) Ronald Ross, formerly of the Indian Medical Service, who devoted himself to this subject during the last years of the 19th century. By a series of experiments he proved that malaria is due to the presence in the human blood of an organism which is conveyed from person to person by this mosquito, and that the mosquito is harmless unless it has become infected with the germ by biting a person who has caught malaria. The value of this discovery was soon shown by practical applications. Major Ross was engaged by the Suez Canal Company to deal with the malaria which had become firmly established at Ismailia, a little town of 10,000 inhabitants on that canal. No fewer than 2,500 cases had been supplied in one year by this small population. The new methods founded on the new discovery proved so effectual that in three years the disease was stamped out, and there has been no relapse ever since. The same results were achieved at Port Said. Now, if malaria is thus caused by mosquito bite, there was some _a priori_ reason for thinking that yellow fever might be transmitted in the same way. At any rate the insect was again laid under a very grave suspicion. The opportunity for studying this further question was afforded during the Spanish-American war, when a serious outbreak occurred among the troops occupying Havana, in Cuba. The doctors were quite unable to deal with this most terrible of all diseases. Knowing nothing whatever of its cause, their treatment of it could be only experimental and casual. So a board of inquiry was formed consisting of four army surgeons serving in Cuba--Walter Reed, James Carroll, Jesse W. Lazear, and Aristides Agramonte. The experiments were begun in June 1900, and continued into the next year. Of these four, Dr. Agramonte was not liable to the disease, and Dr. Reed was called away on duty to Washington. The other two determined to experiment on their own persons rather than risk the lives of other people.