The natural world is jaw-droppingly horrible. Appalling, nasty, vile.
Take trematodes for example.
Trematodes are tiny fish that live in the stomach of a bird. (How did that happen? Horribly. Just keep reading.) They lay their eggs in the bird's stomach. One day, the bird takes a crap into a pond, and the eggs are on their way. They hatch and swim around the pond looking for a snail. These trematodes are microscopic, small enough to lay eggs in a snail's eye, as we used to say in Texas.
Well okay. We never said that in Texas. But trematodes actually do it. For some reason, they always choose the left eye. When the babies hatch, they eat the snail's left eye and spread throughout the body. (Didn't i say this would be horrible?) But they don't kill the snail. Not right away.
First, the half-blind snail gets a gnawing feeling in the pit of its stomach and think it's hungry. It starts to eat but for some reason can ever get enough food. You see, when the food gets to where the snail's stomach used to be, all that's left down there are trematodes, getting their meals delivered. The snail can't mate, or sleep, or enjoy life in any other snaily way. It has become a hungry robot dedicated to gathering food for its horrible little passengers.
After a while, the trematodes get bored with this and pull the plug on their poor host. They invade the snail's antennae, making them twitch. They turn the snail's left eye bright colors. A bird passing overhead sees this brightly colored, twitching snail and says, "Yum..."
The snail gets eaten, and the trematodes are back up in a bird's stomach, ready to parachute into the next pond over.
Welcome to the wonderful world of parasites.