Late last year, Lyn Jolly took a small vial of clear liquid that had arrived in a courier package from Europe, used a pipette to transfer the liquid to the surface of an adhesive bandage, then stuck the bandage to the soft skin near the top of her left bicep.

Over the next 12 hours she felt a bit of stinging beneath the bandage but she left it on – she'd done this before and knew it would be fine once she'd put some cream on it later.

The stinging was caused by 25 tiny larvae of the Necator americanus hookworm, each about half a millimetre long, that were burrowing into her skin, the beginning of an epic journey through Jolly's body that took them through her blood vessels, heart and lungs, then up her windpipe to the back of her throat, where they were swallowed back down her gullet before making themselves at home in her lower intestine. There, over the next few weeks, they grew to their adult size of about 1cm long and the width of a human hair, latched onto her gut lining and started living off her blood.

ISTOCK Meet the Necator americanus hookworm – a menace in the wild, but a medical marvel once domesticated.

There are currently about 180 of these little beasts hanging out in Jolly's gut, merrily mating and producing many thousands of eggs each day, and as far as she's concerned, they are absolutely welcome. She takes care of them, and they take care of her.

Jolly, 57, a human resources manager from Palmerston North, is one of an estimated 6000 to 7000 people around the planet who have chosen to treat themselves with "helminth therapy" – deliberate introduction of parasitic worms or their eggs into the body as a treatment for autoimmune diseases, allergies and an increasingly long list of other maladies.

It's not something your GP will prescribe, but it's far less crazy than it sounds. It turns out that there's a plausible explanation for why controlled exposure to the creatures that can make us very sick can actually make us healthier, and mainstream immunological researchers, including some in New Zealand, are working to pinpoint the mechanisms behind it so they develop new medicines.

Jolly says she already has all the proof she needs. Since installing a hookworm factory inside herself 20 months ago she's seen a dramatic reversal in the autoimmune disease symptoms that were ruining her life.

'I WAS SO SHOCKED'

It was 15 years ago that Jolly first went to her doctor complaining of wrist pain. She assumed it was occupational overuse syndrome but blood tests indicated she had the autoimmune disorder lupus, though at that point she had few other symptoms.

By 2008, though, the disease was starting to bite: she had dry eyes, a dry mouth, diarrhoea and muscle pains. Worst of all was the increasing chronic fatigue.

Her autoimmune diagnosis was refined. She had "undifferentiated connective tissue disease", and there was no treatment available apart from steroids, which have serious long-term side-effects.

Early on, says Jolly, she stumbled across websites where people talked about infecting themselves with parasites, but it sounded ludicrous.

"I was so shocked. It seemed like desperate people trying desperate things."

She muddled along. She saw an endocrinologist and a rheumatologist. She's a sensible, straightforward sort of person but she tried a few things on the border of alternative: acupuncture, Vitamin D, fish-oil supplements. Nothing much worked, and her life was shrinking, especially because of the fatigue.

"Every time you walk past your bed you want to lie down. You start to live an insular life. You don't want to socialise."

She was in her early 50s, but already starting to plan how to exit her career. She wanted to travel, but figured she might have to give up on her dreams.

But then in 2014 a friend recommended a book by the science journalist Moises Velasquez-Manoff An Epidemic of Absence. Jolly read it, and this time she saw the bigger picture. Perhaps, after all, those worm people she'd dismissed were on to something.

The gist is this: Humans and other animals have co-evolved alongside a vast range of bacteria and parasites, creating a kind of arms race between them and our immune system.

But in the past few centuries we've installed sewers to take our faeces far away. We boil and chlorinate water and pasteurise our milk to remove lethal microbes. We wear shoes instead of walking around picking up parasites from the soil. We've invented antibiotics for the bacteria, and antiparasitics for the worms.

These changes are mostly good news, except it seems our immune system had reached some sort of balance with the invader, and with the threats gone, it becomes overactive.

Which is why, the theory goes, the world's richer, more hygienic countries are seeing an epidemic of allergies and autoimmiune disorders that are still virtually unheard of in poorer nations. We've exchanged smallpox, worms and diptheria for hayfever, peanut allergies and lupus.

No one's saying we should give up and let the diseases win, but there is growing interest in the idea of "biome reconstitution" – carefully reintroducing our body to some of the rich variety of lifeforms that used to live on and inside it. Eating a probiotic yogurt to restore gut bacteria after a course of antibiotics is an example.

What caught Jolly's eye, though, was Velasquez-Manoof's advocacy of helminth (parasitic worm) therapy. It seems these worms secrete substances that dampen the human immune response. They're doing it to protect themselves from ejection by their host, but the side effect is a muting of the inflammatory mechanisms driving these new allergies and automimmune problems in humans.

Jolly researched further. She consulted her GP, and her father-in-law, a leading veterinarian pathologist and they both said the same thing: "I've never heard of this, but it makes complete sense."

Jolly joined the Facebook groups where people discuss treatment protocols, the merits of whipworm versus hookworm, and the contacts of suppliers. Twenty months ago, she took her first dose: 25 hookworm through her skin. Five months later, before her colony was even up to full strength, her symptoms started to halt or even reversed. The dry mouth and eyes. The muscle pain. The diarrhoea. It was life-changing.

Best of all: "the chronic fatigue lifted. I actually went for a little jog. I could focus at work."

NO RISK TO COMMUNITY

As a rule, hookworms are bad for you. They cause anaemia, diarrhoea, stomach pain, malaise, weakness, and affect cognition and development. That's why the World Health Organisation is busy trying to obliterate them from the developing world with vast deworming programmes.

Yes, says Jolly, a major infection is bad, but her colony is small and cannot increase in size unless she chooses to. In poor countries infection spreads when people walk barefoot on soil contaminated with faeces from a carrier, completing the parasite life-cycle(see diagram).

"But there's no way you can get it from me, because we have toilets."

Each fresh vial of worms costs Jolly US$200 (NZ$300). But if her own worms are producing eggs every day, couldn't she just poo in her back garden and make her own fresh larvae?

Yes, says Jolly, she could, but she really doesn't want to.

"The early pioneers did, but I don't regard myself as an early pioneer."

The trade in these parasites is a grey area. In the US, says Jolly, the FDA treats the worms as a therapeutic substance, meaning to be legal they'd need to go through the same testing regime as a drug, so packages imported to the US are destroyed if Customs spot them.

Her worms come from a supplier in Europe, and of the four packages she's received three were opened by New Zealand Customs and passed on without incident.

The worm is endemic around the world, so it's likely that far more hookworms arrive in New Zealand in the guts of tourists who've just visited a developing country than in carefully packed vials in courier packages.

Jolly only fears are for the precious menagerie in her duodenum.

"My father-in-law asked me if I would be giving them names, but I hadn't thought about that. But I worry about taking care of them. I had an ice cream sundae last night that had whipped cream, and that would have been made with nitrous oxide."

Nitrous oxide could have harmed them. They also don't like turmeric. Local anaesthesia at the dentist would kill a few, and general anaesthetic is even worse. Pop a worm pill, and they'd all be dead overnight.

Jolly has taken 180 larvae on board, but has no direct way of knowing how many are still alive. There's a blood test that measures white blood cells and a stool test to count the number of eggs: both are proxies for the number of living worms. But like others using helminth therapy Jolly keeps track by assessing her symptoms. If they're returning she's probably low on worms. That was why she did the 25-worm top-up late last year.

The science might be sound, and her results sound great, but isn't it a bit gross infecting yourself with worms that roam through your skin, heart, lungs, throat and gut?

No, says Jolly. "How do you feel about taking a probiotic that's got live bacteria? To me it's exactly the same."

Jolly knows four other Kiwis using helminth therapy, but says the online community was essential for her journey.

In the last year she's noticed people in her closed Facebook group saying, "My doctor said I may want to have a look at this, so I'm here to investigate."

"We have people saying, 'Hey, does anyone know of any clinical trials? I want to do this, but in a controlled way.' And we say, 'Don't have a bar of the clinical trials. You might get a placebo! Stay with us. You'll be OK!'"

Still, says Jolly, "people need to make an informed decision themselves, and understand the research. There's lot of weird stuff out on the internet people need to sift through."

'EFFECTIVE TREATMENT'

Professor Graham Le Gros from Wellington's Malaghan Institute is one of a number of researchers internationally who are trying to understand the finer details the chemical and biological processes allow helminths to moderate the human immune system, in the hope of developing treatments that do something similar, only without the downsides of putting worms in people's bodies.

In a recent statement Le Gros said his investigations were solely at the preclinical stage: "We do not treat patients or carry out research on people"

But last year, researchers at Duke University in the US tapped into the estimated 6000 to 7000 people worldwide who trying helminth treatment. After surveying 250 individuals they concluded that a wide range of inflammation-related diseases, including inflammatory bowel disease, allergies and autoimmunity, were being "effectively treated". Due to the limitations of data from a pool of self-treating patients, the researchers called for "immediate and thorough clinical investigation" of the topic.

Until then, Jolly will carry on regardless.

"Medical research throughout time is full of people doing self-experiments. We wouldn't have blood transfusions if doctors hadn't been experimenting on themselves."

To keep her colony running she needs to keep topping them up as they died. She expects she'll need about 25 new worms each year. The curious things is that when each of miniature medical marvels have done its job it'll just die and move on, and she won't even know it's gone until she notices her symptoms coming back up.

"After a few years I'm going to poo them out, and I'll never see them."

SIDEBAR: A ROUGH GUIDE TO PARASITES

* There are an estimated million species of parasitic worms (helminths) in existence, which infect a wide range of animals including humans. Varieties include flukes, tapeworms, whipworms and hookworms.

* Up to a third of the poorest three billion people on the planet are harbouring parasitic worms. In the developed world parasitic worms are easily treated and far less common.

* Infection with parasites is associated with stunted growth, impaired cognition and increased morbidity and mortality, yet also appears to prevent or improve autoimmune and allergic conditions.

* Eggs of intestinal helminths have been found in the mummified faeces of humans from thousands of years ago

* When opera diva Maria Callas dramatically shed 36kg in the course of year it was rumoured that she had done so by swallowing a tapeworm.