Research in the past couple of years has confirmed what many have suspected: that all our focus on purity and cleanliness may be misguided. Dirty rats, it turns out, are healthier than clean ones, and even intestinal worms can help to prevent and clear up bowel problems. See these articles for more details:
Hygiene Hypothesis: Are We Too Clean for Our Own Good? by Dan Ullrich
Sewer Rats Healthier than Clean Cousins by Seth Borenstein
Can We Be Too Clean?by Jill Fallon
How to Live Long and Prosper: Get Dirty? by Robin Lloyd
War on Bacteria is Wrongheaded by Christopher Wanjek