The Euhaplorchis californiensis is a little parasite who lives in Southern California. And in this movie, released last year by researchers with the USGS, we learn why its lifecycle is the stuff of nightmares.
According to the USGS Western Ecological Research Center:
This parasite infests the brains of small fish in the Carpinteria Salt Marsh near Santa Barbara. It causes a fish to swim erratically, making its host more visible to hungry, predatory birds. When the birds eat the fish and fly away to the sky, they also carry the parasites away, eventually dispersing the parasite's eggs into other ponds through their fecal matter.
But then what happens is even creepier. The dung parasites infest local snails, eating the snails' gonads. Then the castrated snails are eaten by fish, whose brains are eventually controlled and the cycle begins anew. It's amazing how one parasite can do so many completely horrific things. Truly, the Euhaplorchis californiensis is the greatest parasite of them all.
via USGS WERC (Thanks, Ben Young Landis!)