Tapeworm infection is usually pretty chill. You eat undercooked meat that contains tapeworm larval cysts, the tapeworms mature in your intestines, and they hang out there for years taking nibbles of whatever you eat. Like a pet, but on the inside.
(Those nibbles are metaphorical, by the way. Tapeworms actually absorb nutrients through their skin. They also dig into your intestinal wall with hooks and suckers to make sure you won’t poop them out.)
Things can go wrong, though. If you’re already malnourished, those little snacks may be enough to malnourish you even further. (They will probably not cause fashionable levels of weight loss.) And if you have many tapeworms, or if they grow particularly large, they can block your digestive tract.
But by far, the biggest problem is if you manage to eat pig tapeworm eggs. Normally humans eat the larvae in pork, and our pet tapeworms excrete eggs in our poop. Pigs eat the eggs as they’re nosing through filth, and the cycle begins again. But if you, as a human, manage to ingest some eggs (through contaminated water, perhaps) the larvae may take up residence in you. These guys don’t live in the intestines; they make cysts in muscle or, sometimes, your brain. When they do, they can cause seizures. Tapeworm cysticercosis (cysts in body tissues) is a leading cause of epilepsy in places where it’s common.
How to avoid it: Cook your pork, and buy it from inspected sources (pork sold in the US should be fine.) Other animals can harbor tapeworm larvae, but the most significant complications in humans are from pork tapeworms. If you’re in an area where pigs roam freely, make sure to use good toilet hygiene and avoid drinking unsafe water. Boiling water kills the eggs.