At the same time, these groups direct participants to a remarkable amount of scientific information, including numerous peer-reviewed articles. Particularly intriguing are the protocols based on users’ experiences. If you have a terrible reaction to hookworm — which isn’t uncommon — you might start over with smaller doses, and gradually introduce larvae over a longer period of time. Different organisms might also work for different disorders. I spoke with one British scientist, a biologist, who found that hookworm helped an inflammatory condition affecting his liver, but not his ulcerative colitis. Only when he introduced whipworm, which lives at the site of the ulcerations, did his colitis diminish.

At some point, I learned of a private online group where parents discussed treating their children with helminths. After my own experience, I couldn’t imagine intentionally giving my child a parasite, so I was surprised. Many members were mothers; some had immune disorders themselves. A few incubated their own parasites. Everyone I spoke to had given helminths to themselves before their children. Their children had disorders like eosinophilic esophagitis, which can make swallowing food difficult; severe food allergies; and a neuropsychiatric disorder with O.C.D.-like symptoms called Pandas. In several instances, conventional treatment had failed. The parents viewed the potential side effects of parasite infection as milder than the diseases already afflicting their children, and less dangerous than some of the drugs they had already tried. And they claimed to have seen impressive results with parasites.

Some were experimenting with a new organism, a tapeworm native to rats called Hymenolepis diminuta. A British company called Biome Restoration sells it. Depending on the dosage, an order can run just $40. Don Donahue, a radiologist in eastern Tennessee, founded Biome with two other passionate helminths users. He had long suffered from nasal inflammation and obstructive polyps. But just three weeks after acquiring 30 hookworms in Tijuana, he told me, the symptoms improved overnight. “It was like someone slapped me in the face,” he says. He started cultivating them at home for his own use. Spurred in part by the sick people he saw at work, Donahue then decided to make helminths more widely available.

The hookworm Necator americanus (“American murderer”) wasn’t ready for “prime time” in his view. “I really honestly hate hookworm,” he says, because some of the side effects are so bothersome. “But I’m sick without it.” So Donahue settled on what he considered a “gentler” organism, Hymenolepis diminuta. Yet even as he put most of his life savings into starting Biome Restoration, he has kept giving away parasites freely. Of everyone I spoke with, Donahue perhaps most fully expresses the ethos of enthusiastic sharing so often evident among the community of parasite users. I interviewed one young woman suffering from Crohn’s whom he met running at the park and gave a supply of the rat tapeworm. Her Crohn’s eased, she gained weight and, after years of trying, she became pregnant. I spoke with a former snowboarder in Portland, Ore., stricken with a painful autoimmune disorder of the spine called ankylosing spondylitis. That man, Scotty Wittlake — he wanted me to use his full name — told me his condition improved following his self-innoculation with Donahue’s hookworm.

“Citizen scientists are pushing this forward,” says Donahue, who acts more like an evangelist than an entrepreneur. He seems to want to share the miracle he’s found, to spread the word and help others. Biome Restoration has no robust safety data nor any evidence that rat tapeworms help with anything. And parasites that find themselves in unfamiliar hosts can, in theory, cause significant disease and, in rare cases, even death. (In March, the F.D.A. extended an import alert, which already applies to hookworm and whipworm, to tapeworm.) But the company, which seeks to make a more affordable helminth available to more people, is trying something few others have: It is engaging with regulatory authorities to legally sell its product in Britain. The underground is pushing its way aboveground.