IT'S COMMONPLACE TODAY for people to clean their hands with antibacterial or antimicrobial soap, spray their bathrooms and kitchens with bleach and other sanitizers and wash their dishes and clothes at "germ-killing" settings. No doubt, it's an effective weapon for keeping germs at bay. But that doesn't mean we should aim to have our families, including kids whose immune systems are still developing, avoid all germs – if there were even a way to do that. That's because a growing body of research is showing that microbes – microscopic organisms such as bacteria, fungi and viruses – and dirt are...