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272 usually attributed to a mild attack in childhood. If yellow fever has been exterminated from Havana and other endemic centres by anti-stegomyia and other measures, the children in these places can no longer be attacked by the disease. It follows that, in the course of one generation, the entire population will become non-immune. Doubtless, sooner or later the germ of yellow fever will be introduced again, and then, having a population entirely non-immune to spread in, unless promptly checked, a far more extensive epidemic than ever devastated these places in less scientific days will ensue. This can be confidently predicted if it be true that the present immunity of the native depends on an attack of ordinary yellow fever in childhood. I am inclined to think that this explanation does not cover the entire ground; for, if the disease be so mild in childhood as not to be recognizable, how was it that Gorgas stamped out yellow fever in Havana, and how is it that the disease is not in evidence there now? I would suggest that there are two strains of yellow -fever virus one of great virulence, one of little virulence. Specifically the same, they are mutually protective, much in the same way as with vaccinia and smallpox. The immunity of the native and the old resident I would attribute to epidemics or endemics of the mild strain of yellow fever, which, could it be recognized clinically, might be used as a vaccine against the more virulent disease.* Introduction of yellow fever into Asia.— There is another important matter in connection with this disease which, in the near future, ought to be made a subject for international consideration. I refer to the possibility of the spread of yellow fever +987to Asia, the Eastern Archipelago, Polynesia, and