There are loads of varieties of those cordyceps fungi, all specialised to a particular breed of insect. Some species of ants go nuts if they detect an infected member and escort it as far away as possible from the nest to avoid the entire colony succumbing to mushrooms taking over their brains before sprouting out of their heads.
I present to you Cymothoa exigua, a parasitic isopod that preys on the Spotted Rose Snapper fish. It attaches itself inside the fish's mouth at the base of its tongue and starts to draw blood, as the parasite grows, the tongue withers. The parasite then attaches itself to the muscle stumps and acts as a replacement tongue for the fish. The fish can still control it much like it's old tongue but now the parasite gets to get first dibs on anything the fish eats.
It's just so insanely nifty the way it effectively replaces the tongue.
Read Parasite Rex by Carl Zimmer, it's icky but the methods parasites use to overcome their host's immune system are fascinating.
This thread needs: Sacculina parasitic barnacle, toxoplasma and its mind changing abilities, guinea worms, filarial worms, parasitic wasps who lay different types of larvae to defeat the host's immune system and ensure a high female to male ratio in the next generation, and perhaps, if you regard young as being parasites in many respects, the feeding habits caecilians.