For this magnificent work the highest credit is due to the United States chief sanitary officer, Colonel Gorgas. It is well known how the American Medical Commission in Cuba proved six years ago that yellow fever is conveyed from man to man solely and entirely by a gnat common in Central America, known as _Stegomyia_, and further, how by carrying out measures for preventing the entrance of these gnats into dwelling-houses, and especially by keeping them away from yellow fever patients so that they fail to obtain and carry the yellow fever germ, even if they do bite healthy men, Colonel Gorgas and his associates practically eradicated yellow fever in Cuba. The bite of the _Stegomyia gnat_ is the only way in which a man can acquire yellow fever, and the gnat which bites him must have taken up the germs of yellow fever from another man—twelve days (no less) previously. The application of this knowledge and the methods devised to give it effect is what has now rendered the construction of the Panama Canal by the United States Government possible. The French Canal Company employed an army of labourers, numbering from 15,000 to 18,000 men. They lost, almost entirely by death from yellow fever and malaria, so many of their workmen that others refused to undertake the deadly job, and there was a general panic. The death-rate was in 1884 over 60 per 1000. In 1885 it was over 70 per 1000. The work was abandoned. In May 1904 Colonel Gorgas and his forces took possession of the canal zone. This is a zone of territory running fifty miles north and south, with a good-sized town—Colon—at one end of it and another—Panama—at the other end of it. Many hundreds of men were at once organised and set to work to destroy in both the towns the _Stegomyia gnat_. This was effected by doing away with all the breeding-places of the gnat, that is, screening and covering every water receptacle in the town, so that the gnats or mosquitoes cannot breed. Then a fumigating process was carried out in all houses and buildings, great and small, to destroy such gnats as were still alive. No less than 200,000 lb. of pyrethrum and 400,000 lb. of sulphur were used in this fumigation. In December 1905 the last case of yellow fever occurred. It took sixteen months of the work just described to effect this. In a different way the _Anopheles gnat_ or mosquito, which carries the germ of malaria from man to man, was got rid of. This gnat breeds in clean water, where grass and weeds grow; it belongs chiefly to country districts. As it rarely flies more than 200 yards it was sufficient to destroy the breeding pools within that distance of the workmen’s houses, camps, and villages. All the windows and doors of all houses were fitted with wire-gauze screens, which prevent the entrance of the gnats, and the population was furnished with quinin, a dose of 3 grs. a day being ordered to bring the men into such condition that the malaria parasite would not thrive in the blood even if introduced.