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Incidence of various infectious diseases in several Amazon Indian tribes has been determined serologically. Diseases that infect only man fall into two distinct categories. Those which can persist in an individual for a prolonged period are highly endemic, but those which are infectious only in the acute phase die out quickly after introduction. The suggestion is made that the latter diseases could not perpetuate themselves before the advent of advanced cultures and did not exert selective pressures on the human genetic constitution until relatively recently.
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