If you’ve met my mother you might have heard her recommend that children get outside and get in the dirt to build their immunity. Many scientists share her opinion and in medicine, the “Hygiene Hypothesis” and the “Old Friends Hypothesis” explain exactly why this might be true.

Science, like other areas of human intellectual development, tends to progress in a pendular, rather than linear fashion. Take French chemist Louis Pasteur, for example. His theories and experiments lent a ton of credence to the germ theory of disease and his discoveries saved many lives in his and future generations. Fast forward a century and we now live in a society where most people equate germs and bacteria with disease and billions of medical and personal dollars are spent fighting the nasty critters.

The Industrial Age also likely played a part, perhaps as much from moving people away from cow sheds, chicken coops and dirt roads as from the water and airborne pollutants so often blamed for health issues. We’re no longer in regular contact with the harmless microorganisms (including but not limited to helminths, lactobacilli and saprophytic mycobacteria) that have been “old friends” of humans for a very long time. It has been argued that these organisms prime immunoregulation and induce Regulatory T cells, rather than priming aggressive immune responses.

In response to this, and in an effort to stem the rapid increase in human immunoregulatory disorders like allergies, IBD and autism, a growing number of scientists and my mother have good reason to talk us down from the germ theorists’ cliff. Moreover, the hard-to-ignore convergence of two distinct areas of research – the hygiene hypothesis and the effects of pro-and prebiotics are making it increasingly obvious that we have to reconsider the human microbiome.

Two friends of mine sent me interesting emails this weekend on this topic. The first attended an apparently fascinating presentation on the human microbiome at the ACAM conference in Las Vegas and I am looking forward to hearing and sharing more details on what he heard. The second sent me two links to recent and thought-provoking articles, the first in Scientific American and the second in the New York Times. I encourage you to read both.

Sometimes a good thing can be taken too far, like when an innocent crush devolves into stalking. Yes, there are pathogens out there that will ruin your day and end your life if you’re not careful, but no, they’re not all evil. Hygiene and sanitation are good and if you look at it, the improvements in both have changed the lives of billions of people on earth for the better. But taken too far we run the risk of throwing the baby out with the bath water.

Time will tell whether or not we loose the bands of ignorance held fast by good intentions before the pendulum of our own making makes its final pass.