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By Root 802 0

the Hygiene Hypothesis began to fill in a picture of those who were most and least at risk. Children who had lots of brothers and sisters, especially older ones and more especially brothers, who lived on a farm, who went to daycare in their first year or who had a cat were discovered to do better at avoiding allergic diseases than children in none of these circumstances. These factors provided the “dirty” triggers that are needed to spur the ThI lymphocytes into action, thus preventing the Th2 lymphocytes from taking over. Even children whose mother had lived on a farm during her pregnancy were less likely to become allergic. The children most likely to develop allergies and asthma were only children who lived in cities, did not go to daycare, had no pets, washed their hands more than five times a day and bathed more than once a day.

The list of diseases possibly contracted in this way came to include rheumatoid arthritis, diabetes, Crohn’s disease, multiple sclerosis and even heart disease. Rats living in normally germy environments turned out to be less susceptible to arthritis and diabetes than rats raised in germ—free environments. When people emigrated to Europe and North America from Africa, Asia and Latin America, where multiple sclerosis, Crohn’s disease and asthma are extremely rare, their children, who were born and raised in the cleaner developed world, showed the same incidence of those diseases, or even higher, as children with European or North American parents.

The Hygiene Hypothesis remains a hypothesis but an increasingly respected one. There is both contrary evidence (the presence of dust mites and cockroaches has been associated with the development of asthma) and growing corroboration of the theory. So far there are no proven practical applications, although experiments are being conducted in several countries. In Perth, Australia, some asthmatic children are taking a “dirt pill,” with the probiotic bacteria they presumably missed out on as babies and toddlers, and antioxidants. Other children with asthma will receive a placebo, and all will be monitored for frequency of attacks, tolerance for exercise and breathing capacity. Japanese children who were given mycobacteria, a weakened form of tuberculosis bacteria that is related to soil bacteria, were found to have a significantly lower incidence of asthma and allergies than other children.

WISHFUL THINKING

Germs Are Not for Sharing is the title of a children’s book published in 2006 by Elizabeth Vendick that instructs children to play without touching each other. No more Ring-around-the Rosie, holding hands or high-fives.

When I was a child, my playmates and I knew the old saying “You eat a peck of dirt before you die,” but we thought of it as a prophecy, not a command. (There are, in fact, places in the world where folk medicine does prescribe eating clay.) So far, no one has suggested feeding children actual dirt or relaxing hygienic standards to any great extent. The possibility that North Americans and Europeans would continue their usual hygienic routines while taking some kind of dirt pill to beef up their immune systems is a solution Aldous Huxley might have found worthy of Brave New World. Eventually asthmatic and allergic children may take some kind of bacterial medication, but for the rest of us, Tore Midtvedt, a microbiologist at the Karolinska Institute in Sweden and an expert on indigenous flora, advises a more moderate approach. He wants to see an end to “war-on-germs” thinking and an understanding that reflects our often fruitful coexistence with germs. “Find the mechanisms that are at work in those few people that have a disease,” he says. “And use that to eradicate the disease without eradicating the bug.” Midtvedt isn’t advocating that we live close to rats and fleas or drink polluted water, just that we stop trying to live in sanitized houses and bodies. “I’m not saying that we should be more dirty,” he says. “I’m saying we should be less clean.” In other words, we could loosen our currently scrupulous regimen quite a bit before