Free Will Versus Parasites

The lancet fluke changes an ant's behavior so that it grabs ahold of grass blades to get eaten by cattle or sheep -- but only at night. When the morning comes, the ant returns to its normal day-to-day life. (Have you been feeling inexplicably tired, lately?)

This fungus changes an ant's behavior so that it climbs to top of a tree and waits to die. Then it splits open the ant's head to grow out. Cordyceps does the same thing to beetles.

Leucochloridium paradoxum makes snails climb up into the light -- to be eaten by birds.

Nematomorph hairworm makes grasshoppers go jump in a lake.

The Hymenoepimecis wasp injects larva into spiders, and then the larve make the spider weave a cocoon for them, instead of its usual webs.

The Ampulex compressa wasp makes a cockroach into its obedient zombie slave and then leads it to its doom.

The Sacculina barnacle alters the behavior of crabs to make them care for its own eggs, including making male crabs take on the necessary female behavior.

Toxoplasma gondii in cat feces make rats unafraid of cats and affects human behavior too, possibly on a massive (billions of people), culture-changing scale.

Ceti eels enter humans through the ear and wrap themselves around the cerebral cortex, making the victim extremely susceptible to suggestion. Later as they grow, follows madness and death. They can be used as pets by eugenically-bred leaders, though they're still not quite domesticated.