Can gut parasites be good for you? It may sound far-fetched. But for those with off-kilter immune systems, scientists are finding hope in some unlikely allies.

The prevalence of asthma, allergies, and a number of autoimmune-related ills - from rheumatoid arthritis to Type I diabetes - has skyrocketed in recent decades, especially in wealthy countries.

"Roughly four in 10 Americans suffer from allergies, and nearly one in 10 develop an autoimmune disorder," said Duke immunologist William Parker.

Parker is trying to figure out what makes healthy immune systems tick, and why modern living has run them amok.

According to one theory, first proposed in the 1980s, the super-sanitized lifestyle of the Western world may have curtailed some diseases but created new ones.

The culprit, some scientists say, may be a lack of worms. Not the worms that dig in your garden - the minuscule ones that dwell in your gut.

Price to be parasite-free?

Parker is one of a growing number of scientists who wonder if we are paying a price for our parasite-free existence. To find out, he has been studying another animal scrubbed squeaky clean by modern living: lab rats.

Scientists started breeding strains of rodents for laboratory experiments about 150 years ago.

"We treat lab rats with anti-parasitic drugs, and we make sure they have clean drinking water. So in a real sense we've done the same thing to rats that we've inadvertently done to ourselves," Parker said.

About five years ago, Parker began catching wild rats in and around Durham and comparing them to rats raised in the lab.

Unlike sterile lab rats, wild rats are riddled with parasites - not just worms, but bacteria and viruses, lice and mites - which their immune systems have to contend with.

When Parker compared immune reactions in spleen cells of wild rats with their squeaky-clean cousins, the lab-raised rats were over-reactive compared to their wild counterparts.

This could help explain what happens when humans go parasite-free, Parker said.

With parasites out of the picture, the body's natural defenses go into overdrive, such that today our immune systems are mounting the alarm for harmless substances, from dust mites to cat dander.

In the case of autoimmune disorders such as Crohn's disease and Type I diabetes, the body's immune system attacks the very thing it was meant to protect - our own tissues.

"Our immune system doesn't have enough to do," so it gets bored and looks for something to fight, Parker said.

N.C. State biologist Rob Dunn says it makes evolutionary sense.

Dunn is the author of "The Wild Life of Our Bodies," a new book that deals with the creatures that live on and in our bodies, and how our interactions with them have changed over time.

"Few people on Earth were parasite-free until the last century," Dunn said. "Most people in the U.S. no longer have worms, but 50 or 100 years ago that wasn't true."

Parasites found in the guts of ice men and Egyptian mummies tell us that hookworms, roundworms and whipworms have long made their homes in the warm, wet folds of our intestines, bathed in a constant supply of food and nutrients.

Over millions of years of coexistence, Dunn says, our immune systems learned to tolerate these live-in guests, and may have eventually come to depend on them to work properly.

Weighing tradeoffs

If eliminating parasites triggered the rise in allergies and autoimmune disorders, could reuniting with the worms within restore our health?

Some scientists are trying solutions that aren't for the squeamish.

Researchers at the University of Iowa are treating patients with inflammatory bowel disease (IBD) with "cocktails" laced with microscopic whipworm eggs.

It may sound like a witch's brew, but for some patients with IBD - a painful disorder characterized by diarrhea, bleeding and fever - it's a worthwhile tradeoff.

The patients had tried multiple treatments to relieve their symptoms, but nothing worked. After 24 weeks of worm therapy, 23 of the 29 volunteers went into remission.

Worm therapy also has proven effective for other diseases. Multiple sclerosis is a debilitating disease in which the body attacks its own nerve cells.

Scientists in Argentina followed several hundred multiple sclerosis patients for four to six years; during the study, a dozen of them accidentally developed intestinal parasites.

When compared with the patients who remained parasite-free, the worm-infected patients had fewer flare-ups over time.

Today, intestinal worms still infect more than one-third of the world's population. Worms are passed from person to person when microscopic amounts of human feces get on our fingers, or when we walk barefoot on contaminated soil.

But Parker isn't advocating a return to filth. Dozens of communicable diseases, from cholera to typhoid, travel from person to person in human waste.

Sewage treatment and clean running water mean the difference between life and death in some parts of the world, where defecating in the open is a leading cause of contaminated drinking water.

"Nobody's suggesting we go back to the Stone Age," Parker said.

Worm therapy as a cure

Instead, he imagines a future where therapy with certain worms is a routine part of medical care. "You would go to the doctor to get exactly the type and number of worms you needed, then you would get your worm levels checked, just like you get your cholesterol levels checked," he said.

Why not identify the compounds worms secrete, and develop a drug that mimics their effects? Parker is skeptical. "Each worm constantly secretes dozens if not hundreds of different molecules as it travels through the body," he said. "That's hard to reproduce with a drug."

With FDA approval for many kinds of worm therapy still a long ways off, some people are taking their health in their own hands and deliberately infecting themselves with worms in the hopes of relieving their symptoms.

But until we have a better understanding of how worm therapy works, Parker cautions, self-treatment is still a gamble.

"We still need to figure which worm species work best, and how many, and what the timing of treatment should be," he said.

Parker acknowledges worm therapy is a big shift for doctors, who are normally in the business of preventing infection.

"But in controlled doses under medical supervision, the risks are small compared to the potential benefits."