How a handful of Americans got a terrifying, extremely rare disease from raccoon poop

In the summer of 1998, in a small city that juts out into the Monterey Bay, a boy who was not yet 1-year-old began to act very strangely.

He was suddenly irritable, his energy sapped. He seemed to act like a younger version of himself, as if he was sliding backwards in time. This lasted for days.

When his parents took him to the emergency room, they were sent home; it was probably something viral, the doctors said. But that's when things got worse.

The boy became uninterested in his family and increasingly lethargic. His muscles tensed up, his joints seemed rigid, and his right eye looked off-kilter. Something was very wrong.

Just two days after the hospital had dismissed them, the family returned to the ER. This time, the boy was admitted, and later that week, with a fever of 100.8 Farenheit, he was transferred to the University of California-San Francisco Medical Center a hundred miles away.

Doctors there, who later wrote about the troubling case in the journal Pediatrics, finally figured out what was happening. The culprit was a microscopic parasite that's spread by raccoon feces. It's called Bayliscacaris procyonis - also known as "raccoon roundworm" - and once it enters a human, serious symptoms can emerge within days.

The parasite invades the lungs, liver, heart, eyes, and brain and sparks serious inflammation throughout the body. While some people make a full recovery, the infection can leave others blind, in a coma, or with permanent brain damage. It's sometimes fatal.

In North America, the parasite can survive in many different animals, but it's especially common in raccoons, who "shed millions of B procyonis eggs daily in their feces," the Pediatrics researchers explain. Those eggs are extremely hardy, sometimes able to cause an infection even years after they've been expelled.

People who live near raccoons, hunt them, handle them, or keep them as pets may inadvertently ingest the microscopic eggs when they get dirt that's mixed with fecal matter in their mouths, either because they're eating it (often the case with children) or because they neglected to wash their hands.

Those eggs hatch into larvae in a person's small intestine, and then proceed to invade the rest of the body.

Fortunately, Bayliscacaris infection in humans is extremely rare.

As the CDC noted in a September 9 report on seven cases (none fatal) detected between 2013 and 2015, the United States saw just 22 documented cases between 1973 and 2010. Still, given the lack of a commercially available test and the difficulty of making a diagnosis, researchers suspect that some cases may go unreported.