Sept. 18, 2002 -- Behind the ongoing epidemic of allergy and autoimmune disease there's a surprising culprit: cleanliness.

One of the hallmarks of the 20th century was its war on germs. Kids now live in cleaner homes and suffer fewer infections than their grandparents did. There's irony in this, according to the so-called "hygiene" theory. It holds that a germ-free childhood warps the immune system. This may lead not only to allergic diseases but also to autoimmune diseases such as type 1 diabetes and multiple sclerosis.

A report in the Sept. 19 issue of TheNew England Journal of Medicine now offers powerful support for this theory. The researchers carefully vacuumed up dust from the beds of 812 children from rural areas of Austria, Germany, and Switzerland. The dust was measured for a basic component of dirt -- the outer cell wall of common bacteria, known to scientists as endotoxin. Also collected were the kids' medical records.

The result: the kids who had the cleanest mattresses had the most hay fever, allergic asthma, and allergic reactions. The kids with the dirtiest beds -- and least allergy and asthma -- were most likely to live or play on farms. That's no surprise, as earlier studies showed that children raised on farms have fewer allergies and less asthma than rural kids who don't live on farms. Bacteria excreted by cows and other farm animals are the most common source of endotoxin.

These findings support previous research showing that having a cat or a dog in the home as a child or attending day care during the first year of life helps prevent the development of allergies and asthma in kids.

"Farm kids just have a natural environment, and this suggests that if you have natural exposure to endotoxin, it is helpful," study leader Charlotte Braun-Fahrl