
Life is rough for parasites. Say you’re a tapeworm that only lives in the gut of one species of shark. You start out as an egg inside an adult tapeworm. Your parent releases you and a bunch of other eggs from its body, and its shark host shoots you out of its own body. Now you float in the vast ocean, stretching out on all sides. You are not made for the free-living world. If you don’t get into another host, you will never reach adulthood. Not just any host, but a fish. And not just any fish, but one species of shark. Chances are good, in other words, that you’ll die.
The miserable odds for individual parasites can potentially drive the evolution of something remarkable: the ability of parasites to manipulate their hosts. By controlling their hosts, the parasites can raise their odds of surviving and reproducing.

