Ribeiroia ondatrae is a parasite that turns frogs into monsters and feeds them to birds.
If you thought creatures like the Guinea Worm and Leucochloridium were creepy, (see: www.cracked.com/article_17199_7-most-horrifying-parasites-on-planet.html) here's one whose hideous symptoms are so outrageous that nobody thought they could be natural; for many years, horribly "mutated" frogs repeatedly made headlines as a sign of man's impact on the ecosystem. The media was certain that these multi-limbed abominations were the spawn of nuclear waste, pesticides, mercury and mad science.
While pollution is still a serious problem and threatens millions of species around the world, these "mutant" frogs have probably been around before we even discovered coal, though we're not quite sure if the reality is better or worse than atomic sludge.
Inhabiting freshwater ponds and wetlands, Ribeiroia begins its life in the gullet of an aquatic snail, multiplying asexually into swimming sperm-like monsters who sniff out the nearest tadpole and drill their way inside. Here, the worm decides to play God (or possibly Faust) and fuck with the tadpole's transformation into an adult frog.
By encysting itself in the cells that usually grow a tadpole's legs, the parasite upsets the growth process and causes the host's limbs to develop in more chaotic numbers, angles and locations, creating gimpy, malformed freaks who can scarcely move without tripping all over themselves.
Not all biologists have reached a consensus on why these parasites have made such a dick move in their evolution, but the commonly accepted hypothesis is that the deformed frogs have a harder time fleeing or hiding from predatory birds, the parasite's final host, which will spread its eggs around in their feces. Explanations we would have accepted just as readily include "Nature has a fucking awful sense of humor" and "IA! IA! CTHULHU FHTAGN!"