The gym bag is a perfect spot for germs to hang out. Warm, damp clothes, sweaty shoes and wet flip-flops -- a surefire recipe to grow bacteria that can result in colds, flu and even infection. Don't sweat it, simply clean it up.
Whether you've got a wet swimsuit or sweaty gym clothes, put them in a plastic bag or outside in the gym bag's mesh area. Hang things to dry when possible so mold doesn't multiply.
Re-useable water bottles can be teeming with germs, especially around the spout. Wash after each use with hot soapy water, especially those with pull-tops. Think about getting a screw-cap bottle to protect yourself.
Yoga mats get germy from both you and whatever is on the floor. If it's your mat, disinfect after class or wipe it down when you get home.
Studies show the shower drain has the most germs in the gym. So your flip-flops worn in the shower, sauna and locker room can pick up all kinds of fungi and bacteria. They too should be put in a plastic bag before going into the gym bag.
Sneakers, too -- disinfect with cleaner or sanitary wipes and let them air out before repacking.
Finally, realize the gym bag itself is going to be crawling with icky stuff. Regularly empty it and air it out. Try not to put it on the gym or the bathroom floor. Instead, hang it up or put it on a towel.