Infection begins as snails grazing on algae incidentally ingest worm eggs. The eggs hatch into worms that prevent a snail’s own reproduction. Instead, the infected snail nourishes the growing larval worms, which eventually develop into a free-swimming stage and leave the snails to seek their second, or intermediate, host—the California killifish. In traveling to the fish’s brain, the worm causes the fish to behave differently from other killifish; it moves about jerkily near the water’s surface. This behavior attracts predators like herons. The heron, in turn, becomes the host to the adult worm. The adult trematode takes up final residence in the bird’s gut, releasing thousands of eggs that are deposited by way of bird droppings back into the salt marsh, completing the parasite’s life cycle. How might such a complex life cycle have evolved?