Humans may hold dominance over most other life-forms on Earth, but a few varieties of organism have long held mastery over us. Ironically, these life-forms, including bacteria and viruses, are so small that they cannot be seen, and this, in fact, has contributed to their disproportionate influence in human history. For thousands of years, people attributed infection to spiritual causes or, at the very least, to imbalances of "humors," or fluids, in the human body. Today germ theory and antisepsis—the ideas that microbes cause infection and that a clean body and environment can prevent infections—are ingrained so deeply that we almost take them for granted. Yet these concepts are very recent in origin, and for a much longer span of human history people quite literally wallowed in filth—with predictable consequences.

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