Say hi to
Cymothoa exigua, otherwise known as the tongue-eating louse. These guys enter into a fish's mouth through the gills and eat the fish's tongue.They then proceed to latch onto the stub and act as the fish's new tongue. It feeds on what the fish consumes in addition to occasionally sucking the fish's blood. They then proceed to get busy in the fishes mouth before releasing eggs into the surrounding water.

Yeah, this parasite ranks pretty high on my nightmare list (thankfully it isn't attracted to humans).