It's NOT a threat to your child for a bit of contact with harmless everyday substances such as germs, dust and dirt! I fully agree that playing outdoors and getting dirty is a completely normal thing to do for children even without wearing slippers, but being sensible of course to the extent permitted.
There is no reason why you should be terrified of your child playing and crawling on the floor out of fear that he/she will be exposed to some dirt. This in fact could actually lead to some health benefits. Many of us know it, but some of us still refuse to believe it.
Exposing your little ones to germs and animals can boost their immune system and, as immunologists believe, are less likely to develop asthma, Crohn's disease, diabetes and even heart problems.
Having inadequate exposure to bacteria, many children nowadays are growing up cleaner than a normal person should be, which can result to under developed immune systems. Diseases that have now become more common include diabetes, inflammatory bowel disease, multiple sclerosis and rheumatoid arthritis - all these have been linked to making the immune system over-react if a person becomes overly clean.
A professor of biology and author of The Wild Life of Our Bodies, Rob Dunn, urges to adopt an approach to the "hygiene hypothesis", in which he believes that the future lies in "re-wilding our bodies" by convincing ourselves that the human body is still in the natural state of our ancestors who used to live in the wilderness without any immunizations and sometimes live in unsanitary conditions. In short, if people long ago survived in such circumstances and have lived for several years, then it is possible to have foreign substances enter our bodies and still develop into healthy human beings.
But this whole concept of exposing your child should begin at a very early age in life as this helps the body prepare to develop antibodies, which helps to fight off infections as opposed to getting sick often.
Helpful Resource:
Yahoo
My Personal Opinion