Scientific American has a slideshow up of various different viruses, parasites, and fungii that turn poor innocent, and sometimes cute, animals into brainless zombies.

From the article:

Crickets can’t swim, but the harrowing hairworm (Spinochordodes tellinii) doesn’t care about that detail. After growing inside of a cricket’s body and feasting on its insides, the hairworm will inexplicably compel the cricket to throw itself into a body of water, where the ruthless body snatcher can emerge and enter the aquatic phase of its life cycle. “It was amazing to see hundreds of crickets at night totally under the control of the parasite inside and jumping into the water,” says Frederic Thomas