Mad science: Zombie animals.

Scientific American, a magazine whose title increasingly sounds like a deliberate and almost provocative anarchronism (like the CP in NAACP), has a nifty article and slide show on behavoir-changing parasites: parasites that not only infect a host, but hijack existing behavoirs and twist them to better serve the spread of the parasite. Here's two examples from the article:

In the case of the spooked spider (Plesiometa argyra), a parasitic wasp (Hymenoepimecis argyraphaga) lays her eggs on the spider's abdomen. Just before the larva emerges, the host spins a strange, new type of web—one that looks nothing like its usual wide nets. This silk platform, however, is perfectly suited to supporting a cocoon for the vulnerable young wasp larvae, which have been feasting on the spider's innards as they grow.

The snail-manipulating flatworm (Leucochloridium paradoxum) grows and multiplies inside the snail. Once ready to move on to its next host, the worms push up into the snail's tentacles, making them swell and squirm, mimicking the action of bugs that birds like to eat. As the snail crawls, blindly, into the sunlight, a passing bird is likely to swoop down to snatch a tasty tentacle or two. The worm-infested meal will then infect the bird, which passes it onto other snails via dubious droppings.