You hit the gym regularly to be healthy and fit. Yet you may be getting more than flat abs and strong arms there. Gyms are hotbeds of germ activity, researchers say. Norovirus, which causes stomach pain, vomiting, and diarrhea, can survive for a month on the surface of exercise machines. The fungi responsible for foot infections multiply at a blinding pace in the shower. And microbes like MRSA, an antibiotic-resistant bacteria that can lead to dangerous skin infections, lurk in the locker room.