You've seen the recent reports and scratched your head in wonderment: After years of scientific consensus that cleanliness is next to wellfulness, evidence is accumulating, like mildew between the shower tiles, that the opposite may be true.

First came the news that having two or more cats or dogs around to slather your infant's face in salivary affection appears to help a baby build up immunity to allergies and asthma.

Then in September, researchers showed that hanging out with farm animals may help shield kids from allergies. After that came a study indicating that having older siblings and going to day care seems to provide the same type of allergy and asthma protection as consorting with pets and livestock. (For study details, see Page F4.)

And so we're faced with the emerging conclusion that, after many years of laboring under the illusion that a pristine environment would protect kids from unseen pathogens, regularly exposing them to dirt laden with bacteria will make them healthier.

Scientists are still reluctant to recommend raising hogs in the spare bedroom or skinny-dipping in the septic tank as preventive medicine. The recent research, though voluminous and credible, is still considered inconclusive. And much is not yet known about the precise relationship between filth and human health.

Let's begin with the big picture -- or big pictures, since the scientists who study this are not in agreement about why allergy and asthma immunity happens at all in these settings.

One school of thought holds that the traditional "tolerance theory" is at work. This suggests that early exposure to specific allergy triggers changes the way the immune system responds to them. And that faced with the same allergens later on -- the dander of a slobbersome golden retriever, for instance -- the body knows better than to respond with an allergic reaction. Tolerance theory is very specific; scientists who hold to it say a person develops an immunity to only one allergen at a time.

Others prefer the "hygiene hypothesis," which holds that developed nations are too clean and that our relentless efforts to wipe away grime are messing with our immune systems, which need a bit of squalor to function at full tilt.

Spearheaded by scientist David Strachan in the late 1980s, the hygiene hypothesis claims that exposure to bacteria, viruses and other microbes early in life helps fortify the developing immune system against a broad array of potentially allergenic substances. Without such exposure, we become vulnerable.

Dennis Ownby, chief of the Allergy and Immunology section at the Medical College of Georgia in Augusta and author of the recent study about dogs and cats, said the hypothesis just makes sense, adding that other recent studies support it.

"We have done a great job with public health in terms of sanitation, clean food and water, but now live too clean a life," said Ownby. "It's hurting us."

All this cleanliness started a century ago, Ownby explained, when nations such as the United States began establishing cleaner sewer systems and learning how to purify water and properly clean food. The incidence of tuberculosis plummeted. So did cases of other infectious diseases. Then, in the 1950s, vaccines came onto the scene to eradicate infections that led to measles and other such communicable conditions. All of that accounts for about 90 percent of our cleanliness today, said Ownby. The remaining 10 percent or so can be attributed to our more recent obsession with things antibacterial.

Our higher health standards correspond with an exponential increase in allergy and asthma among children. One controlled double-blind study done last year strongly upheld the hygiene theory, said Ownby. In it, researchers in Finland identified a beneficial bacteria that had been part of humans' intestinal flora prior to the 20th century. Researchers fed the bacteria, now mysteriously absent from the human gut, to pregnant women and then to the infants after they were born. The babies experienced increased immunity to atopic eczema, allergic rhinitis and asthma. The researchers concluded that improvements in hygiene and a reduction in family size might explain the bug's disappearance.

At the center of the hygiene hypothesis is endotoxin, a chemical found in the outer cell wall of common bacteria. Proponents of the hypothesis are fairly sure that endotoxin is the magical substance that stimulates a portion of the immune system, thereby reducing one's risk of becoming allergic.

The bacteria that sport endotoxin come primarily from the gastrointestinal tracts of mammals. Some of the most common sources live in the mouths of dogs and cats and the excrement of cows and other livestock. It gets into your body through breathing or (hopefully accidental) swallowing.

Others aren't so sure about the endotoxin angle. Scott T. Weiss, a professor of medicine at Harvard Medical School who recently wrote an editorial on the topic for the New England Journal of Medicine, said more work needs to be done to pin down whether other factors in the environment may be involved. For example, he asked, what substances interact with endotoxins to stimulate the immune system, and does one need to be infected by an endotoxin, or just exposed?

"I think this is an area where we're going to be making a lot of progress -- but it's still too early to know what's going on," cautioned Weiss.

Other scientists are wary of the hygiene hypothesis altogether.

"It just doesn't tie everything together. Like all theories in medicine, there are a lot of inconsistencies," said Robert A. Wood, an associate professor of pediatrics at the Johns Hopkins University School of Medicine. Wood was lead investigator of a study showing that mouse allergen, in the form of urine or dander, is widely distributed in the inner city and may be a significant contributing factor to the childhood asthma epidemic in urban areas.

Wood said inner-city populations don't experience the protective effects of exposure reported in many of the recent studies on allergy and immunity. And under the hygiene hypothesis, he said, they should -- since their conditions are not unusually clean (not unlike the farm settings) and involve many people packed into a small space (not unlike the day-care environment). But asthma and allergies among inner-city groups, he said, are "off the scale."

Clean Is Good, Too

Though the hygiene hypothesis holds that getting rid of infectious microbes has weakened our immunity apparatus, the upside of such eradication cannot be overlooked, says Marshall Plaut, chief of allergic mechanisms at the National Institute of Allergy and Infectious Diseases, a unit of the National Institutes of Health.

"Reduced exposure to microbial products is not a stupid idea," said Plaut. "It's a good idea sometimes and an absolutely crucial idea at other times. Is there a level of exposure that would protect us from bad infections but wouldn't change our likelihood of developing immunity to allergies? That's something we don't know yet."

Plaut said he wonders whether the children in the recent studies got infections that were just not part of the data set. And if so, he asked, what's better -- a less-than-tidy environment that protects from asthma but is rife with infection, or an infection-free zone that may promote allergies?

"We don't know if they [the participants in the studies] could have other problems that could be worse, or long-term complications from infections," said Plaut.

Other experts have problems not with the theories but with the studies themselves. Some were surveys, not controlled scientific studies, these critics say. They lack sufficient detail in some cases.

More Dirt Needed

Since no scientists are quite ready to recommend a retreat from high standards of hygiene based on the recent research, all are waiting for more highly refined studies to be completed. Ownby said he and researcher Christine Johnson just got a grant to expand their dog-and-cat research. They plan to choose 3,000 children to study early next year. Meantime, Weiss and researcher Fernando Martinez are attempting to sequence the genes that may be involved in the allergy-immunity process. Also, Weiss is looking at diet and how fatty acids and antioxidants may affect immunity to allergy and asthma.

Once these and other projects are completed over the next five years, researchers are hoping to develop a vaccine of sorts that would ensure immunity to allergies and asthma without requiring families to live with 101 Dalmations. Supporters say that's about 10 years away.

For now, researchers urge parents to not do anything drastic, like consider day care a form of disease prevention or farming a health insurance plan.

And they are especially hopeful that parents don't run out to get a pet for children who are already asthmatic or have allergies. This will do more harm than good, said Wood.

That said, the thinking has changed when it comes to what doctors advise for parents who are wondering whether to get a pet. Instead of telling them to steer clear of pets for fear of allergies in their babies, Wood now tells parents the evidence is unclear; even he acknowledges it may sometimes be beneficial to have a dog or cat that occasionally gives the baby a kiss.

And if the hygiene hypothesis hold true, what sort of soaps should you use to wash the baby and clean the house? Any kind you want.

"What kind of soap you use is not going to make a difference one way or the other," said Weiss.

For now, scientists are careful to refrain from saying much more.

"I just don't think we know enough yet for me to say, 'Take two dogs and call me in the morning,' " said Weiss. "On my desk, I have a picture of my two sons standing on a giant pile of manure in hip boots, visiting their babysitter's farm," said Weiss. "But I don't think any of us are ready to make public health recommendations that people go do the same thing on a regular basis."

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Suz Redfearn is a regular contributor to the Health section.

Several recent studies suggest that living with, if not like, farm animals can boost your immunity to disease. One study shows that people who attended day care or had many siblings have fewer allergies and less hay fever. Some scientists believe exposure to pets' endotoxins, microscopic poisons on the outer walls of bacteria (shown as wavy lines at the bottom of the diagram at right), strengthens the immune system.