As someone who writes a lot about evolutionary biology, I’ve often had people say to me, “I just can’t believe that evolved.” Originally, that referred to the lovely side of nature–the beauty of flowers, for example, or the grace of birds in flight. The implication was that these things were so beautiful and intricate that they had to be created for a purpose–a beautiful purpose, obviously.

But after I started writing about parasites, that underwent a fascinating change. Parasites may be deadly and gross, but they also have some mind-boggling adaptations. Most mind-boggling of all is the way many species travel through two or more hosts during their life cycle. Some flukes live first in snails, which cough them up