Most of us have heard of tapeworms, the parasitic creatures that find their way into the human digestive system, and can grow very long indeed. They can cause quite a bit of trouble, but I had no idea how extreme the trouble can sometimes be.

Theodore Nash sees only a few dozen patients a year in his clinic at the National Institutes of Health in Bethesda, Maryland. That’s pretty small as medical practices go, but what his patients lack in number they make up for in the intensity of their symptoms. Some fall into comas. Some are paralyzed down one side of their body. Others can’t walk a straight line. Still others come to Nash partially blind, or with so much fluid in their brain that they need shunts implanted to relieve the pressure. Some lose the ability to speak; many fall into violent seizures.

Underneath this panoply of symptoms is the same cause, captured in the MRI scans that Nash takes of his patients’ brains. Each brain contains one or more whitish blobs. You might guess that these are tumors. But Nash knows the blobs are not made of the patient’s own cells. They are tapeworms. Aliens.

This is scary—they can find their way into the bloodstream, and in the human brain, where they happily live and grow as cysts.

Well, let’s back up a bit. I didn’t know that the tape worm life cycle involves humans and pigs, and that the normal life cycle can only be completed via undercooked pig meat. There you go, I thought, that’s why you should avoid undercooked meat.

But—no. That’s not the half of it.

The more serious trouble (of the brain cyst kind) happens when the normal tapeworm cycle is disrupted. Instead of finding their way inside a pig, tapeworm eggs sometimes find their way straight back inside humans, and the confused eggs behave as they would in a pig—reach for the blood stream. And that’s the recipe for disaster. You could be having tapeworm cysts in your brain, without ever having had raw or undercooked meat.

I won’t give everything away; go read the whole article. It’s excellent, informative, and as I said, a little scary.