Are intestinal worms the next probiotic? Some scientists think so. A growing body of research suggests that infection with helminths, or parasitic worms, may help balance the immune system and reduce inflammation. Some studies suggest that the use of helminths—including hookworms and whipworms—can ease the symptoms of a growing number of medical conditions.

The U.S. Food and Drug Administration (FDA) disagrees. The agency has classified helminthic therapy as an Investigational New Drug (IND). The only people who can dispense INDs are researchers conducting experiments, and even they need special permission. But treatment with the parasites is legal in Mexico, and would-be hosts are crossing the border to get it.

There's clear desperation at work here: A person who is willing to intentionally infect him- or herself with intestinal worms—which affect 2 billion people worldwide and can cause a range of serious illnesses—likely believes they've already exhausted every other option.

So why would anybody do this? 

Autoimmune and inflammatory diseases are on the rise in the U.S. and other wealthy countries. With conditions like allergies, type 1 diabetes, multiple sclerosis, and lupus, a person’s body actually attacks itself. And for many of these people, existing treatment options are just not working. 

Scientists believe that the increase in autoimmune diseases is related to the increase in cleanliness and sterile environments. In order to learn to protect us from all the little nasties out there, the bacteria in our bodies have to meet those nasties. But in a world with antimicrobial hand soap in every public restroom, our bacteria aren’t meeting much of anything. In the absence of challenges, our bacterial ecosystems can get a little lopsided, with too much of certain types of organisms. And, researchers say, that imbalance can send our immune systems into a tailspin. In short, we’re too clean for our own good. 

This hygiene hypothesis, as it’s known, is also the idea behind probiotics. If we supplement our “good” bacteria, the theory goes, we may keep things in check. And some researchers believe the same could be true for helminths. Infection with the parasites activates the immune system in a way that might provide sick people with some relief.

There are a number of ways to take in the parasites. Visitors to helminth therapy clinics may be offered a shot of clear, helminth-infested liquid to drink; the worms might also be applied to their skin. When it comes to hookworms, swallowing the worms sends them quickly to the gut, where they dig in to the stomach lining and begin feeding. Worms that enter the body through the skin first have to enter the bloodstream, wind their way into the heart, and then the lungs. Because the worms irritate lung tissue, some people cough them up at this point, then swallow them again. One way or another, the worms then migrate into the small intestine, where they feed.  

Helminth therapy recipients have reported decreases in allergies, inflammation, and pain, but it's hardly instantaneous. “The use of the intestinal worms is typically not a short term solution,” notes the website of a clinic called Worm Therapy. Some people may begin to feel the effects within a few weeks. For others, it may take months or even years. And for some people, it will never work at all.

Science seems fairly split on this one. There are plenty of studies suggesting that helminth therapy can be safe and effective, but most conclude that we just don’t have enough information yet to be sure. 

Gastroenterologist Joel Weinstock has performed a number of helminth experiments on patients with Crohn’s disease. “Nobody got hurt, nobody’s eyes fell out,” Weinstock told Science Notes in 2014. “But it’s still too early to say, ‘Well golly gee, this is going to be better than apple pie.’”