Most people think germs on their bodies are a Bad Thing. If they didn't, hand sanitizers, disinfectant soap and bleach would not sell so well. As we learn more about the microbes that inhabit our bodies, we are realizing that the human being is really a superorganism composed of about a trillion human cells and 10 trillion microorganisms. Rarely a microbe will cause illness, but these are transient residents that the body actively removes. Most other microbes that live on us are benign and a significant number of them are critical to our good health.
In the latest issue of Science magazine, Torsten Olszak et. al describes how the microbes we come in contact with as children stimulate natural killer cells (part of the immune system) assisting in their development. Without this contact, the natural killer cells do not develop tolerance and can precipitate immune-related diseases such as irritable bowel syndrome and asthma. This contact must occur during childhood for tolerance to develop.