But in the last 150 years, the industrialized world's clean food supply and plumbing suddenly removed parasites from people's bodies. In response, researchers now widely think that people's immune systems stopped developing properly.
Weinstock said most people still have a powerful "attack" function of their immune system, but that many believe the immune system does not develop to regulate properly in the absence of helminthes (parasitic worms).
"People who are not exposed to helminthes have sloppy regulation," said Weinstock. As a result some people's immune systems go off kilter and misfire against their own bodies creating autoimmune disorders such as allergies, asthma, or inflammatory bowel disease.
The parasite argument ties in to the so-called "hygiene hypothesis" from the 1980s, and has been in development in renowned scientific journals, such as The Lancet and Nature, ever since.
In the case of adults, Kuhn pointed out, the immune system is already developed and cannot prevent reactions like asthma and allergies. But researchers have noticed that parasites still may have an effect on those suffering from some disorders.
"About 10 years ago, many of us studying parasites realized that parasites could induce the immune system to do odd things," said Kuhn.
For example, Weinstock and colleagues thought that patients suffering from inflammatorybowel disease were missing a chemical in their intestines that helped control inflammatory responses. When a parasite was introduced, the body made the chemical to fight it, and the inflammatory bowel problems began to clear in most patients.
But similar discoveries haven't convinced doctors of the merits of treatments sold and distributed without monitoring under the FDA.
"What is their clinical proof that it's useful?" asked Kuhn. "If they're selling this stuff, they've got to have serious science backing it up."
Medicine widely available in the United States can easily kill the beef tapeworm (Taenia saginata) sold by Worm Therapies and the whipworm (Trichuris trichiura) and hookworm (Necator americanus) sold by both Autoimmune Therapies and Worm Therapy.
But Kuhn pointed out that a parasite is a parasite, and can cause sickness.
"There's a huge movement going on to try and find a vaccine for hookworm throughout the world," said Kuhn. "Infected children and pregnant women can become particularly anemic."
Dr. Henry Milgrom, professor of pediatrics at National Jewish Health University of Colorado in Denver, agreed with Kuhn.
"I have heard of such things. So this is not completely off the wall, and it is possible that parasite infestation can protect against allergic disease. Having said that, I don't think that to infect oneself with parasites without some studies showing that this is safe and effective is reckless," said Milgrom.
"What [parasites] do is they suck blood and cause pain and diarrhea and congestive failure. My guess is they probably pick some more benign forms," said Milgrom. "But you cannot view this in any way than any other treatment that you are giving people."