We have followed Dr. Reed now up to the point of his having infected mosquitoes and being ready to transmit the disease to non-immune human beings. A human being, in order that he may be liable to yellow fever, must be non-immune, and by immune I mean a person who either has had yellow fever, or has lived ten or more years in a locality where yellow fever prevails. An attack of yellow fever gives a great immunity to the disease, probably just as much as occurs in the case of smallpox. In practice, it is counted as absolute. In over two thousand cases of yellow fever which I have treated personally or seen in consultation, I have never seen a single case with a second attack, in which I saw the same individual in the first attack. I have seen several, however, who believed that they had had a previous attack, and I myself believe that I saw them in their second attack. I have by no means seen a quarter as many cases of smallpox as I have of yellow fever, yet I have seen more cases of second attacks of smallpox than I have of yellow fever. I feel confident, therefore, in stating that yellow fever gives fully as great immunity as does smallpox. It is well known that in a yellow-fever endemic center such as was Havana during the nineteenth century native Havanese are not liable to yellow fever. They look upon their immunity as being absolute, and in my experience of fourteen years of life in such endemic centers I am inclined to accept their belief. The immunity of the native is explained by saying that he has probably had yellow fever in childhood when the disease was very mild, and that, at the time, it was overlooked and not recognized. This is the best explanation that, so far as I know, can be made of the facts in the case. Certain it is that one of these endemic centers from which yellow fever has been banished for a number of years may have yellow fever as badly as a city in which it has never been endemic. Eighty years ago a native of Mobile, Alabama, or Pensacola, Florida, looked upon himself as being as immune to yellow fever as did the Havanese twenty years ago. But at the present time the native of either of these cities is just as liable to yellow fever as is the man from New York. This is explained by the fact that eighty years ago they had yellow fever so frequently in Mobile and Pensacola that all the natives had this disease in childhood. Within the last fifty years they have had it so infrequently that very few now living in those cities have had this disease.