Until now, the rules of hygiene were simple. People whose standards of cleanliness were lower than yours were disgusting. People whose standards of cleanliness were higher than yours were obsessive. The clean freaks had the moral high ground, and manufacturers were really good at marketing to the fear of grime.

Nearly 700 new antibacterial products -- lotions, toys, mattresses, touch screens -- hit the market in the U.S. between 1992 and 1998, and the bandwagon shows no signs of slowing down. TV shows about the germiness of motel bedding or the germiness of raw chicken have made a vivid impact. I've noticed that increasing numbers of perfectly sane people won't leave home without a bottle of an antibacterial hand cleaner like Purell, so they can sluice down hourly.

The citizens of Japan are even more concerned about hygiene than Americans. (Accordingly, stories marveling at their obsessiveness are a favorite of American media coverage of Japan.) Japanese manufacturers pump out antibacterial telephones, scissors and deposit books. One company claims to sell a million antibacterial ballpoints a month. Some department stores have whole sections of antibacterial stationery. My favorite product: antibacterial bait.

Sanitation has been a triumph of modern medicine, all but eradicating many diseases and parasites. We've whipped the body louse, trichinosis, typhoid and childbed fever, and we're not a bit nostalgic. But there are signs that this very preoccupation with good hygiene is making some of us sick.

Many immune system disorders are on the rise. There we sit, in our sparkling kitchens, using spotless glasses of pure filtered water to wash down antihistamines. The number of people complaining of allergies has doubled in the past decade. The number of people with asthma (currently around 17 million Americans) increased by 46 percent between 1982 and 1993. Other immune disorders, from Crohn's disease to multiple sclerosis, are also proliferating. For instance, the incidence of Type 1 diabetes, an autoimmune disease, is increasing by 3 percent to 5 percent a year. While many of these illnesses have a genetic component, the rises are too sudden to be the result of genes spreading through the population.

Could it be that our laudable cleanliness has something to do with the increase in immune disorders? Epidemiologists, immunologists, bacteriologists and parasitologists from England to Iowa think this may be the case. According to what's called the "hygiene hypothesis," our immune systems, which evolved in environments where we couldn't escape disease, microorganisms of every description and just plain dirt, don't always develop normally if they don't meet these things during our childhood development.

Exactly how might such environmental challenges as bacteria, viruses and other microorganisms work to set the immune system on the right track? Most proponents of the hygiene hypothesis say that there is a balance in our immune systems between two kinds of reaction, the Th1 and the Th2. Th1 lymphocytes primarily respond to bacteria and viruses, and Th2 lymphocytes primarily respond to parasites. Th2 is in charge of classic allergic symptoms such as releasing histamines and releasing sheets of mucus to wash parasites away.

Many hygiene hypothesis proponents suggest that if a baby's developing immune system doesn't meet enough viruses and bacteria, or if it doesn't meet the right kind, Th1 reactions may not be sufficiently stimulated. The immune system may become unbalanced, with an overactive Th2 response, which treats an innocent flake of cat dander as a menacing parasite and makes the person in question feel dreadful. The end result? Classic allergic symptoms: itchiness, sneezing and maddening quantities of mucus.

Dr. Koichiro Fujita, a parasitologist at the Tokyo Medical and Dental University and author of many books on hygienic Japan, including "Cleanliness Is a Sickness," is concerned about his country's love affair with antibacterial goods. Fujita says this fetishistic sanitation is giving the Japanese allergies, dermatitis, asthma and generally out-of-whack immune systems.

"We human beings used to possess immune cells to deal with outside germs and parasites," Fujita told Newsweek. "But the cleanliness boom took away the job of those cells. They became unemployed. That is why they began to react to such items as pollen and ticks."