We may not realize it, but each one of us is a walking ecosystem. Minuscule, eight-legged Demodex mites nestle head down inside the follicles of the eyelashes, feasting unnoticed on skin cells. Microscopic yeasts live on the tongue, teeth, and skin and in the intestine. Dormant viruses like herpes simplex may loiter for years inside nerve cells. Perhaps strangest of all are the self-replicating, viruslike pieces of DNA that infected ancient humans and still make up about 8 percent of our genome.
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