Back in the bad old Stone Age, humans had to put up with all sorts of creepy crawlies. Parasites ran amok in people’s innards, freeloading on nutrient supplies. The parasites took a toll, but over the millennia, those that killed off their meal ticket too quickly didn’t make it. The survivors of this evolutionary shakeout include parasitic roundworms and flatworms, hitchhikers that allow their human host to live on — and to provide three hots and a cot.
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