Nineveh came with her own case of tapeworm, making her two pets in one, or many many pets in one if you count each tapeworm segment, or proglottid, as its own animal. I was, naturally, horrified and had her treated right away, but I can't help but kind of admire the feline tapeworm's navigation of its complex life cycle. It's pretty unusual for a parasite to require an intermediate host. In the absence of a useful Ctenocephalides felis -- that's science for "comb-headed cat thing" -- or other bloodsucker, the tapeworm (Taenia taeniaeformis, which is science for "flat band shaped like a flat band," and I guess I can't argue with that) can hang out in the organs of prey animals like rodents and wait until cats eat them. One way or the other, the final host must swallow the tapeworm eggs -- and yes, a human can get infected with T. taeniaeformis, if she happens to swallow an infected flea -- or eats an infected rat, I guess. Humans do that stuff a lot less often than cats do.
Often, feline tapeworm causes no symptoms at all except for the host's shedding egg-bearing proglottids. The real reason for treating it is that it's so comically disgusting to humans! The way the medication for it works is kind of cool too: it paralyzes the worm so it loses its grip on the cat's intestinal wall and is expelled. The natural world and our responses to it are fascinating, even when they're gross! Humans are pretty smart, as it turns out. We can figure out how to do stuff like paralyze a worm without hurting the cat it lives in. Of course it's also a pretty good trick that the tapeworm can live inside a cat in the first place without hurting it, but we did our thing on purpose.