In last week's issue of Science, a group of researchers from Harvard Medical School reported that mice raised with more bacteria were healthier than those raised in more sterile conditions. Bioethicist Art Caplan wrote up a nice summary of a paper here.

The scientists reached an admittedly geeky conclusion: "These results indicate that age-sensitive contact with commensal microbes is critical for establishing mucosal iNKT cell tolerance to later environmental exposures," they wrote in the journal Science. In other words, exposing baby mice to common germs got their immune systems appropriately busy and able to not over-react when encountering nasty bugs and other biological stuff later in life.

This is a big deal.

The rapid rise in food allergies, asthma and other immunological diseases is due, at least in part, to our modern obsession with cleanliness, scientists increasingly believe. The 'hygiene hypothesis', first advanced in 1989 by the British epidemiologist David Strachan, contends that these diseases are becoming more common because young children are not exposed to them at an early age. We spend so effort trying to prevent exposure to germs with antibiotics, antibacterials and soaps that letting kids get dirty seems like a violation of basic parental duty.

Caplan agrees with the authors that the experiment supports the so-called hygiene hypothesis, which blames today's rise of autoimmune disease and allergies on a lack of germs. That's still controversial but what's clear here is the message that doctors and medical researchers should be considering the human body as a product of evolution.

Our immune systems evolved under very different conditions from those people face today, and there hasn't been much time to adapt. We are also co-evolving with all kinds of microscopic symbionts and parasites. Not all microbes are our enemies. We couldn't survive without some of them.