Parasites and Allergies: What the Hygiene Hypothesis Gets Right and Wrong

In places where gut worms are common, allergies tend to be rare. That observation helped fuel the “hygiene hypothesis,” the idea that fewer early-life microbial exposures leave the immune system more prone to allergies. But when researchers tested it, removing worms didn’t cause allergies, and giving worms to people with asthma didn’t cure it.

This guide explains what the parasite trials actually found, what research says about related early-life factors like dogs and fish, and why experts now say “hygiene” is the wrong word. For more, see our living with allergies guides.

Jump to: Quick answer · The idea · Parasite trials · Dogs and farm animals · Fish in infancy · Is hygiene to blame? · FAQ

Quick answer

• Deworming didn’t cause allergies: in a trial of 1,566 Vietnamese schoolchildren, treating worms raised the chance of a positive allergy skin test but didn’t increase asthma symptoms, rhinitis or eczema.
• Worms didn’t treat asthma: a small trial that infected 32 adults with asthma with hookworm found no significant improvement.
• Dogs in the first year were linked to slightly lower asthma risk at age 6 in a study of about 1 million Swedish children; farm animals showed a stronger link.
• Regular fish before age 1 was linked to less allergic disease by age 4 in a Swedish cohort.
• Don’t stop cleaning: experts say everyday hygiene isn’t the cause of rising allergies and still prevents infections.

Where the idea came from

The term “hygiene hypothesis” traces back to a 1989 BMJ paper by David Strachan titled Hay fever, hygiene, and household size, which linked larger families to less hay fever. Since then, the idea has broadened: researchers now focus on exposure to a range of microbes in early life, including those in the natural environment and the human microbiome, rather than on dirt or infections alone.

Parasitic worms (helminths) became part of the story because, as the Vietnam trial authors noted, observational studies suggested worm infection protects against allergic disease and allergen skin sensitization.

What the parasite trials found

So worms may dampen positive skin tests, but a positive skin test is not the same as having allergy symptoms. The trials don’t support deliberately getting or keeping a parasite infection to prevent or treat allergies. Parasite infections carry their own health risks, and treatment decisions belong with a doctor.

Dogs and farm animals

A 2015 study in JAMA Pediatrics used Swedish national registers covering more than 1 million children born from 2001 to 2010. Living with a dog in the first year of life was linked to a lower risk of asthma at age 6 (odds ratio 0.87, about 13% lower odds). Living with farm animals was linked to a larger reduction (odds ratio 0.48).

Keep in mind:

• This was an observational study, so it shows a link, not proof that dogs prevent asthma.
• It looked at early-life exposure in babies. It doesn’t mean a dog will help someone who is already allergic to dogs.

If you’re thinking about a pet and someone at home has allergies, see are hypoallergenic cats real?

Fish in a baby’s first year

In a Swedish birth cohort of 4,089 infants followed to age 4, families with allergies or babies with early eczema or wheeze tended to delay fish. After excluding those children, regular fish before age 1 was linked to a lower risk of allergic disease by age 4 (adjusted odds ratio 0.76) and of allergic sensitization. Only 18 of 2,614 tested children were sensitized to fish.

Like the dog study, this is observational. Talk with your child’s doctor about when and how to introduce fish and other allergenic foods. For more on early diet, see our guides to raw farm milk and allergies and diet and allergies.

Is hygiene to blame?

A 2016 expert review in Perspectives in Public Health concluded that the term “hygiene hypothesis” is a misleading misnomer. The authors found evidence that contact with environmental microbes and the human microbiome helps regulate the immune system, and that urbanization, diet changes and antibiotic use have changed our microbiomes. But they found no good evidence that hygiene, as the public understands it, is responsible.

Instead, they suggested factors such as natural childbirth, breastfeeding, social contact, outdoor activity, diet and appropriate antibiotic use, with a focus on early life. They also recommend “targeted hygiene,” a risk-based approach that protects against germs that cause infections while still allowing the spread of helpful microbes between family members.

This guide is general information, not medical advice. Read our medical disclaimer.

Frequently asked questions

Do parasites protect against allergies?

Observational studies suggested they might, but a large trial in Vietnam found that removing worms raised positive allergy skin tests without increasing actual allergic disease. A trial giving hookworm to adults with asthma found no significant benefit.

Is the hygiene hypothesis true?

Partly. Early-life microbial exposure does seem to shape the immune system, but experts say everyday hygiene isn’t the cause of rising allergies and recommend keeping targeted hygiene to prevent infections.

Does having a dog reduce allergies in children?

A Swedish study of more than 1 million children linked living with a dog in the first year to slightly lower asthma risk at age 6. It’s an association, and it doesn’t help people already allergic to dogs.

Does eating fish reduce allergies?

A Swedish cohort found regular fish in the first year of life was linked to less allergic disease by age 4. It’s observational, so talk to your child’s doctor about introducing fish.

Should I get a parasite to treat my allergies?

No. Trials haven’t shown a clinical benefit, and parasite infections carry health risks.

Sources

• Flohr C, Tuyen LN, Quinnell RJ, et al. Reduced helminth burden increases allergen skin sensitization but not clinical allergy: a randomized, double-blind, placebo-controlled trial in Vietnam. Clin Exp Allergy. 2010;40(1):131-142.
• Feary JR, Venn AJ, Mortimer K, et al. Experimental hookworm infection: a randomized placebo-controlled trial in asthma. Clin Exp Allergy. 2010;40(2):299-306.
• Fall T, Lundholm C, Örtqvist AK, et al. Early exposure to dogs and farm animals and the risk of childhood asthma. JAMA Pediatr. 2015;169(11):e153219.
• Kull I, Bergström A, Lilja G, et al. Fish consumption during the first year of life and development of allergic diseases during childhood. Allergy. 2006;61(8):1009-1015.
• Strachan DP. Hay fever, hygiene, and household size. BMJ. 1989;299(6710):1259-1260.
• Bloomfield SF, Rook GA, Scott EA, et al. Time to abandon the hygiene hypothesis. Perspect Public Health. 2016;136(4):213-224.

Last reviewed: October 21, 2026.