Remember that kid on your block that ate dirt; 'accidentally' falling into the Mud Pond or slogging through swamps as a kid ? Even though you swore you washed your hands before meals it was a crap shoot. Relax, ancient humans did all of that and worse. It was of course, early preparation for Mountain Biking but there were ancillary benefits. It turns out that your body didn't like that nasty stuff any more than your Mom did, so it learned how to measure and react to the onslaught of germs.
20Th century medicine gave us the impression that clean places are the ultimate answer for our health, Hospital clean. Americans believe that we should Sanitize every chance we get. Spray it, wipe it inoculate it, anything to kill the nasties. Unfortunately we know less than we think we do about germs. One dangerous place people go today is the local hospital. Staff infections, VRSA (Vancomycin-resistant Staphylococcus aureus) and a host of other organisms have evolved past our ability to wipe them out. Doctors over per scribe medications then bugs mutate faster than we can react. The Center for Disease Control estimated 94,000 drug resistant infections and about 19,000 deaths (ref -1, -1.1, -1.2) from these super bugs in 2005. From what I've read its going to get worse before it gets better.
Humans have dealt with over a half million years of gunk (ref -2), so that's a good track record. "All Things in Moderation" means that you have a quota for swallowing that glop of mud that just whipped off of your front tire into your mouth. Its even OK to eat fruits and vegetables that don't have the perfect complexion shown on TV. Our immune system was born to learn and repair. Go to the doctor when you're sick, stay healthy by riding. Good diet & exercise are responsible for saving more lives than sanitized wipes ever will. It has been clinically proven that the smile you get from descending The Luge from the Santiago Truck Trail, kicks the crap out of all kinds of stuff that is out to get you.
The next time u see some kid in trouble for getting into a mess, just smile and wink, they will know what you mean :).