Allergies Genetics Hygiene Hypothesis

The hygiene hypothesis suggests something that, a few years ago, would have been medically unthinkable: that our obsession with hygiene is actually making us sick. There’s mounting evidence that the body’s immune system has to be exposed to minor threats early in life in order to understand how to function properly.

The Origins of the Hygiene Hypothesis

The hygiene hypothesis is not new. Dr. David Strachan proposed this theory in 1989, after he noted large families have lower-than-normal rates of asthma and allergies. Dr. Strachan suggested that children in larger families were exposed to more infections through their siblings, resulting in healthier immune systems that were less likely to mistake harmless substances for allergens.

Since Dr. Strachan’s initial hypothesis, further studies have built on the theory that being too clean may be bad for us. At least one study suggests that exposure to pet dander in early life actually reduces levels of adult allergy and asthma symptoms.

Researchers speculate that a baby’s tendency to explore the world by mouthing everything may help program the immune system, allowing it to tell the difference between harmful and harmless substances. Or as Dr. Joel Weinsotck, director of gastroenterology and hepatology at Tufts Medical Center, Boston, puts it, at birth the immune system is “like an unprogrammed computer. It needs instruction.”

Intestinal Worms and Hygiene

Researchers have suggested that the immune system is like a muscle. If it isn’t properly stimulated, it won’t work properly. Bacteria and viral agents, however, don’t seem to be as important to a healthy immune system as intestinal worms.

Intestinal worms are common in developing countries such as Gambia. Research has uncovered that autoimmune system diseases such as asthma, Crohn’s disease and multiple sclerosis are extremely rare in such countries.

When families from countries such as Gambia move to cleaner, more hygienic developed countries, however, something changes. The first generation born in the supposedly “healthier” developed nation has no contact with intestinal worms, but develops chronic immune disorders at a rate equal (or sometimes even higher than) families that have lived in developed countries for generations. The theory is that microorganisms that live on the intestinal worms may help strengthen the immune system by giving the system something to defend against.

Further evidence for the hygiene hypothesis can be found in studies of farm families. Children raised on farms have lower rates of immune disorders than their urban counterparts. Researchers suggest this may be due to exposure to organisms in farm animals and manure that stimulate the immune system from an early age.

The Impact of the Hygiene Hypothesis

The hygiene hypothesis has yet to be proven, and no one is recommending letting children put whatever they like into their mouths. The hypothesis does suggest, however, that we may not be doing children any favors by over-shielding them from disease and dirt. Some researchers suggest that family pets, far from being potential allergens, may help stimulate the immune system by exposing children to mild organisms and infections.

Immune system researchers believe the hygiene hypothesis may point the way to new treatment methods. A university of Iowa study deliberately infected patients with inflammatory bowel disease with pig whipworms, which only survive in the human intestine for a week. All six patients eventually had their disease symptoms go into complete remission.

If the hygiene hypothesis is correct, medical science may one day treat chronic immune disease not by suppressing the immune system, but by deliberately challenging it.


Brody, J. (2009). Babies know: A little dirt is good for you. Retrieved March 23, 2009, from the New York Times Web site: