It's called the "hygiene hypothesis," and it was first proposed in 1989. The idea is that exposure to a variety of bacteria, viruses and parasitic worms early in life helps prime a child's immune system, making him less susceptible to certain diseases later on. Without exposure to these microorganisms, the immune system may not know what to do when it comes across certain foods or bacteria ... leading to allergies to foods, pollen and pet dander. In worst-case scenarios, it may even turn on the body's own tissue, setting off autoimmune disorders.

Allergies and autoimmune diseases were virtually unknown in the U.S. before the turn of the last century, but health experts say they began springing up as modern sanitation, decontaminated water, food refrigeration and antibiotics became more widespread. In 1998, about one in five children in industrialized countries suffered from allergies and asthma according to the International Study of Asthma and Allergies in Childhood. But such diseases are still relatively rare in rural communities of Africa and Asia, as are Type 1 diabetes and multiple sclerosis. In other words, these are diseases that have emerged hand-in-hand with our over-washed hands.  

Exposure to immune-stimulating microorganisms may also lower the risk of future illnesses such as heart disease. In a study of 1,700 Filipinos followed from birth to age 21, those who grew up around chicken, pigs and dogs and had bouts of diarrhea in childhood had lower rates of C-reactive protein, an inflammation marker that has been linked to cardiovascular disease, as young adults.

But that's not to say that we should let our kids drink from muddy streams and eat dirt. After all, there is a reason that all of that hand-washing became so popular. According to a recent article in the Wall Street Journal, nearly 70 percent of the 8.8 million deaths of children under age 5 worldwide in 2008 were caused by infectious diseases, most frequently pneumonia, diarrhea and malaria. And infant mortality rates are still far greater in rural "developing" communities than they are in industrial communities — 31 per 1,000 in Namibia and 34 per 1,000 in Mongolia, compared to seven per 1,000 in the U.S. and three per 1,000 in Japan.

The key now is to find a way to harness the immune-protecting effects of microorganisms without the risk of exposure to fatal diseases. Clinical trials in the U.S. and Europe are testing Trichuris Suis Ova (TSO) — a species of pig whipworm — as a treatment for peanut allergies, ulcerative colitis, Crohn's disease and MS. It's also being tested with asthma and even autism.  

In the meantime, should you and your kids stop washing your hands? Absolutely not, but don't be afraid to let your kids play in the dirt, either. And hey, maybe that five-second rule isn't such a bad idea after all.

Dirt: It does a body good
Is the modern uber-clean lifestyle a hazard to your family's health?