FRIDAY, Sept. 9 (HealthDay News) -- Good hygiene has saved millions of lives, protecting people from countless bacterial and viral infections, according to the U.S. Centers for Disease Control and Prevention.

But there is growing concern that strict adherence to good hygiene, though a valuable means of protecting health, has left humans open to other forms of illness.

Proponents of the "hygiene hypothesis" believe that reduced exposure to bacteria, viruses and parasites have impaired the immune system's ability to properly respond to environmental challenges.

Researchers have identified the hygiene hypothesis as a possible cause or exacerbating factor in a number of illnesses and medical problems, said Dr. Graham A.W. Rook, a professor in the department of infection at the Centre for Clinical Microbiology at the University College London. These include:

• Severe allergic reactions.
• Gastrointestinal disorders, such as inflammatory bowel disease and Crohn's disease.
• Autoimmune disorders, such as type 1 diabetes and multiple sclerosis.

"The evidence for all this is very, very powerful," Rook said. "It's very easy to show if you live on a farm or keep a dog, you're less likely to have these disorders. If you are the youngest child in a big family, you're less likely to have these disorders."

The hygiene hypothesis has its roots in the theory of evolution, he said.

"The bottom line is organisms that were present in mud, untreated water and feces were with us right from the start of humanity," Rook explained. Proponents of the hygiene hypothesis believe that the human body adapted to these organisms and began using them as a means of training the immune system.

"What has happened over the course of evolution is, because these bugs had to be tolerated, they came to activate the tolerance of the immune system," Rook said. "They are the police force that keeps the immune system from becoming trigger-happy. Basically, the immune system is now attacking things it shouldn't be attacking."

Dr. Mitchell H. Grayson, an associate professor of pediatrics at the Medical College of Wisconsin in Milwaukee, said that the hygiene hypothesis is most strongly linked to an increase in allergic diseases and asthma.

"It's thought to have something to do with the way your immune system develops and is programmed," Grayson said. Bacteria in the environment teach an immune response to allergens that is more moderate and less severe. "In the absence of these bacteria, the immune system is thought to become more prone to allergic disease," he said.

Rook said that other researchers have used the hygiene hypothesis to show that parasitic infections can aid in the treatment of such conditions as multiple sclerosis and Crohn's disease.

Argentinean researchers, for instance, have shown that the presence of intestinal parasites can moderate the progression of multiple sclerosis. Follow-up studies indicated that, when people were treated for their parasitic infection, they had a relapse of MS.

A research team at the University of Iowa found similar results related to Crohn's disease, showing that intestinal parasites helped regulate the autoimmune reaction that causes the intestinal disorder.

On the flip side, such revelations carry risks. Doctors are concerned that some might use the hygiene hypothesis as an excuse to abandon good hygiene, causing a surge in diseases such as dysentery and cholera.

"Public health and sanitation has been the single greatest improvement in our life expectancy," Grayson said. "I would not recommend living less cleanly."

On the other hand, tolerance of a little dirt here and there won't hurt.

"If your kids come back from the garden with a little mud on their hands, it's not a bad thing," Rook said. "They don't necessarily have to wash their hands before picking up a sandwich."

In general, though, it's wise to maintain good overall hygiene, he said, and wait for the scientists to figure all this out. Larger studies are underway to determine the exact mechanism by which bacteria and parasites are causing the immune system to moderate its response, Rook said.

"We need to figure out how to replace what is good from the microbiological environment while maintaining the advances of good hygiene so we can get the best of both worlds," Rook said.

More information

The U.S. National Institute of Allergy and Infectious Diseases has more on the role of microbes and bacteria in sickness and health.