Yellow fever is an acute specific, very fatal, febrile disease, lasting about a week, and characterized by fever, vomiting, muscular pains and albuminuria, and in the graver cases by black vomit and hematogenous jaundice. It is transmitted from person to person by the female stegomyia mosquito. The sick person does not infect the biting mosquito after the third day of the disease. One attack gives immunity against a second attack. Like all other diseases, its origin is enveloped in a cloud of obscurity. The yellow-fever organism, like the horse or the dog, developed in some particular locality. The horse had his whole lifetime in which to wander from place to place, and so spread very rapidly and widely as compared with the yellow-fever organism. The yellow-fever organism was dependent upon the mosquito and man for his locomotion. On the average, he had not more than a week in which to travel. I presume that a week would be long for the average life of a stegomyia mosquito, and the traveling ability of the organism would be limited by the life of the mosquito. For while the female stegomyia mosquito has to live two weeks after she has bitten the yellow-fever patient before she can convey the disease to another non-immune human being, still the average length of life in the mosquito cannot be as great as this. The various conditions of unfavorable environment, the natural enemies of all kinds, wind, etc., must kill a large proportion of mosquitoes within the first week. If we consider the total number of stegomyia mosquitoes in an infected locality, we must see that only an infinitesimal proportion succeed in biting a yellow-fever patient within the first three days. This small proportion, however, has the best protection and is more likely to have a long life than the average of the female stegomyia, for they necessarily bite inside of the houses, and in such places have the best protection from the wind and sun, their greatest enemies. In this other host, man, the parasite had only three days to travel. During these three days the man would be sick and not likely to move about much. During man’s savage state he traveled very little more than did the mosquito. We would expect, therefore, the yellow-fever organism’s rate of spread to be very much less rapid than the spread of the horse. When America was discovered, the horse had not yet reached that continent. The evidence with regard to the yellow-fever organism seems to me to indicate that, at this same time, it had not spread further than a limited area about Vera Cruz. It is probably the latest disease to which man has been subject. There are very many beliefs as to the locality at which yellow fever developed; that it originated among the earliest nations of the world inhabiting Asia Minor; that it originated in Africa in connection with the slave trade; that it originated in America, and was not known to Europeans until the discoveries of Columbus.