Helminth therapy is a new frontier in experimental medicine, promising to cure all manner of conditions from allergies to autoimmune diseases to irritable bowel syndrome to even, possibly, autism.

Here’s the catch: It is not yet recommended or approved by the FDA and there is very little interest in the treatment from members of the established medical community. This means anyone wanting to try it out for themselves will have to sign onto a clinical trial (which are sporadic and small in number), travel abroad, or look to the grey market and the unregulated, often unscrupulous companies offering helminth therapy online.

So what does this encouraging – if deeply controversial – therapy actually involve?

The short answer: intestinal worms.

Patients purposefully inject or ingest intestinal worms (aka helminths) or their larvae to reboot their immune system and calm symptoms of inflammation-related conditions. The theory goes that, in order to survive inside a human host, a helminth must regulate the body’s immune response and, by doing so, prevent it from overreacting to internal and external stimuli – the physical process behind autoimmune diseases.

It sounds icky. After all, most of us have been brought up to think of intestinal worms as parasites and a harbinger of disease, sickness, and, in some cases, death. Yes, contracting worms can trigger a range of unpleasant symptoms, including diarrhea, abdominal pain, malnutrition, and fatigue. It can also impair growth and physical development in children, and in the worst case scenarios be life-threatening. Yet, the right type of helminth in the right quantities may be beneficial to its host – and perfectly safe.

“Helminths are a lot like bacteria,” William Parker from Duke University’s Department of Surgery told IFLScience. “Some of them can kill you and some of them are beneficial.”

Helminth advocates like Parker refer to the “Old Friend Hypothesis”, which postulates humanity has spent the majority of its existence living alongside benign microbes – and that includes non-harmful parasites and helminths. We’ve co-evolved over tens of thousands of years. In essence, we and they are “old friends”.

Humans have spent hundreds of thousands of years co-habiting with intestinal parasites like the hookworm. The eggs (picture) are found in contaminated soil where they can be picked up by people walking around barefoot. toxinmaster/Shutterstock

Only recently, following the economic prosperity and technological boom kick-started by the industrial revolution, we have lost our old friends. Modern society is too clean. At least, it is in the West. More than 1.5 billion people worldwide are infected by soil-transmitted helminths, the majority in sub-Saharan Africa, the Americas, China, and East Asia, and when worm eradication programs do take place, studies show that rates of autoimmune diseases spike in the immediate aftermath.

“[Helminths] are a natural part of our environment that we’ve lost and they really need to be put back into everyone for disease prevention, we believe, not just for disease treatment,” said Parker.

Parker, who has spent years studying intestinal worms’ effect on the human immune system, sees helminths as a therapy rather than a drug. He argues we will need to “reconstitute our biomes” by re-introducing helminths to our body’s natural eco-system.