About 80% of all infectious diseases are transmitted by both direct and indirect contact, says Philip Tierno, the director of clinical microbiology at New York University Medical Center and the author of "The Secret Life of Germs." That makes the gym, with its sweaty bodies in close proximity, a highly conducive environment for catching everything from athlete's foot to the flu. In swabs of medicine balls, for example, Tierno found samples of community- acquired MRSA, a strain of staph resistant to some antibiotics. "You take your chances," Tierno says. "Any time you touch a medicine ball or machine, you have to know that your hands are contaminated and should be washed." What about those spray bottles that some gyms provide for wiping down equipment? They may help, Tierno says, but he recommends additional measures, such as wearing long sleeves and pants while working out. Also, bring your own towels because there's no guarantee that your gym's linens have been bleached or rinsed in clean water. While in the locker room, make sure you wear flip-flops, and avoid sitting nude on any exposed surface.