YOU ARE HERE BECAUSE OF YOUR ANCESTORS’ IMMUNE SYSTEMS. IF any of them—as tree-dwellers of hunter-gatherers on the plains of Africa, or as farmers or herders in Bronze Age villages, or during the great epidemics of civilized history—had succumbed to any of the many microbes that ruthlessly cull humanity, then you would not be reading this right now. Somehow, because of better nutrition or greater intelligence or geographic circumstance or, most likely, just plain dumb luck, whatever ailments, diseases, and infections your predecessors were stricken with weren’t fatal, and those forebears successfully reproduced.