The doctor educated me. Broadly speaking, there were two types of tapeworm that could be my roommate. There was your basic pork tapeworm, and your basic beef tapeworm. Eat some not-so-well-cooked pork or beef and you could get some eggs inside you that your warm, moist innards would nurture and hatch and help the parasite infants grow big and strong, but, for the pork parasite, only for a few meters inside the intestine.
However, your beef tapeworm was an altogether mightier alien. In came cow food slightly undercooked, and eggs-hatch-grow, but the beef worm could grow more than just a few meters; it would keep adding to its body, lengthening and snaking through the miles of intestine. I have it on good authority that, with the small and large intestines combined, it could reach the moon, loop it twice, then come all the way back down to earth and you'd still have enough left over to play skip rope with.
This beef tapeworm would fill all this out until it eventually ran out of intestine upon reaching the rear exit. My beef-based house guest had grown so big, so happy, so healthy, had filled me up so much that it now had to drop little bits of itself in the toilet bowl every few days just to let me know how great he was doing.
Diagnosis: I was pregnant with a parasite that had started its life going moo.