Brain parasites
The tapeworm larvae form little nodules in the brain like this one.
A California woman suddenly lost feeling in her legs, then the rest of her body. Her doctors couldn't figure out what could be causing the numbness, until they took a look in her brain with a CT scan. The scan showed that calcified tapeworm larvae were lodged in her brain.

The diagnosis, officially called Neurocysticercosis, is common in the third world (about 50 million people are infected globally). But American doctors often don't know the symptoms of the 1,900 cases that show up in American hospitals every year.

Mollie Bloudoff-Indelicato describes this brain parasite in a Scientific American guest blog post about the California woman, named Sara Alvarez:


They can grow to be 20 feet long and can make tens of thousands of eggs a day.

The rates seem to be pretty low, but the case described in the blog post demonstrates how much difficulty American doctors have diagnosing it, when they don't know what symptoms to look for.

The worms are originally tiny: about the size of peas. They come from eating contaminated pork, and fight their way into the small intestines and cling to the flesh there while draining nutrients from their host. They can grow to be 20 feet long and can make tens of thousands of eggs a day, which ride through the bloodstream through the whole body.

The large circle indicated by the black arrow is a sucker, the dark dots highlighted with green arrows are calcified larvae.
They turn into larvae but don't grow any bigger in humans (in pigs they do), so these babies stay small and colonize tissues throughout the body. From Bloudoff-Indelicato's blog post:

Scientists aren't quite sure how it works, but tapeworm larvae seem to have developed a chemical secretion that keeps the human body's immune system from barging in on their banquet. People can live for decades without any symptoms of neurocysticercosis because the tapeworm larvae break down natural defenses. Unfortunately, tapeworm larvae can't live forever.

"While it's alive, it's a problem, but when it starts to die it's a bigger problem," [Columbia University scientist Dickson] Despommier says.


Alvarez had to have brain surgery to get the worm out. Lots more info on the wormy brain buddies over at Scientific American.

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