Exercise is great for your immune system, but the gym may not be if you don't pay attention. Bacteria and other microbes love the moisture of sweat and shower stalls, and although gyms do what they can to make the environment as clean as possible (i.e. by making hand sanitizer and clean towels available) it's ultimately up to you to protect yourself. Wash your hands frequently, wipe seats and handles with a clean towel (not the one you've been carrying around with you from machine to machine) or a disposable wipe, and bring your own mat for stretching and yoga sessions. There are even handy gadgets out there like these HandleBuddies, designed to stand between you and the germs.

It might seem like a hassle, but it's worth it. You can catch more than just a cold if you're not careful -- even bacteria like MRSA and E.coli have been found on gym machine handle bars.