This is FRESH AIR. I'm Terry Gross. You may be shocked to learn what's living in your home. The bacteria, the fungi, viruses, parasites and insects - probably many more than you imagined. But if that makes you think you'd better start scrubbing right away, my guest Rob Dunn says the real goal is to create a healthy balance of all the microbes that co-habit with us. You might be upsetting that balance with some antimicrobial sanitizers.