Parasites are typically seen as dangerous, as well as disgusting: The nastier ones can gnaw off a fish’s tongue, hijack an ant’s brain, or cause human limbs to swell to grotesque deformity.

But don’t recoil just yet. Parasites can also have some beneficial impacts on human hosts. Here’s how:

They may boost fertility

Being infected with roundworms might make a woman more fertile, a study published last year in the journal Science suggested.

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Biologist Aaron Blackwell of the University of California, Santa Barbara, looked at nine years of data from nearly 1,000 women in a Bolivian village with parasitic infection rate of 70 percent.

The women in this village, who didn’t use contraception, had an average of nine children. It turned out that those infected with a type of roundworm, known as helminths, got pregnant earlier and had their children closer together. The reverse happened — fertility was impaired — for women infected with a different parasite, a type of hookworm.

The researchers believe that parasites trigger immune responses that can affect how likely a woman is to successfully carry a fetus to term.

They might bring relief from allergies

Spring brings days of sneezing, itching, and sniffling. One theory is that our hyperclean modern society has greatly reduced human exposure to parasites. Consequently, the immune system doesn’t have much to do except pick fights with proteins from peanuts or pollen.

Could allergy sufferers find relief by getting a few worms under their skin?

Researcher John Turton tried this in the 1970s. A chronic allergy sufferer who worked for the Medical Research Council in the United Kingdom, Turton intentionally infected himself with parasitic hookworms. He later reported in the Lancet that his allergic reactions were reduced for the two years that the parasites lived inside him.

While more extensive study has yet to bear this out, scientists are investigating.

They may reduce symptoms of irritable bowel disease

Just this week, researchers reported in Science that parasitic worms reduce symptoms of irritable bowel disease in mice. These worms basically help the small intestines keep irritating bacteria at manageable levels.

The team found that chronic infection with a parasite similar to roundworm “restored the mucus and cell morphology within the small intestines.” They also suggested that IBD is becoming more common as parasitic infections wane in the developed world.

They could help heal wounds

Liver flukes produce a type of protein called granulin, which can cause the unchecked proliferation of cells that mark cancer. (Indeed, people infected with flukes are more likely to develop bile duct cancer.)

But rapidly multiplying cells also can close wounds. In fact, as the fluke eats through its host’s liver, it heals the wounds it creates with a growth factor it produces.

Last year, scientists found that this growth factor accelerated wound healing and even seemed to help grow blood vessels. It could potentially be used to heal diabetic ulcers and surgical incisions.

They might ease symptoms of multiple sclerosis

A clinical trial underway in Europe is investigating whether certain parasitic infections could protect against multiple sclerosis.

The trial was inspired by an earlier study from Argentina that found people with MS who were naturally infected with gut parasites experienced much milder symptoms of the disease.

One theory is that the hookworms trigger regulatory T cells, which keep inflammation under control. Or it could be that the parasites induce changes in the gut flora that somehow mitigate the symptoms of multiple sclerosis.

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