Avoiding illness at the gym is a bit like driving defensively: You need to anticipate that other people will make mistakes, leading to dangerous (read: germ-y) situations. But while most of your attention is focused on the germ-ridden elliptical machines and free weights, you're completely overlooking a sneaky, disease-harboring source: your gym bag.

"Eighty percent of all infectious disease is transmitted by contact," says Dr. Philip Tierno, Jr., professor of microbiology and pathology at the NYU School of Medicine. That can be from either direct contact or indirect contact (like picking up your gym bag and them touching your eyes, nose or mouth).

"Where there are people, people impart their germs. And that's where you get the pathogens," Tierno says. "With a little caution and awareness of how we spread germs, I think you can break the transmission." Translation: It's time to grab your gym bag and start scrubbing. Here, the germiest items hiding in your gym bag, and the nasty pathogens that have taken up residence.

If you bring your own towel to the gym, you're off to a good start. But you need to be careful about how you store it. Always put used towels in a plastic bag before sticking them in your gym bag, or you could infect everything inside of your bag with pathogens like E. coli.

Michael G. Schmidt, Ph.D., professor and vice chair of the department of microbiology and immunology at the Medical University of South Carolina, says that people literally ooze E. coli germs. "It falls out of people," he says. (And yes, he means that in exactly the way you're picturing.) "Think about the behaviors of people in the gym," he says. "They're wet, and it seeps out and contaminates the bench." Even through a towel. "The liquid helps to wick the microbes out of its normal residence in our colon," he explains.

Then, if you sit down on the bench, you can pick it up on your towel or skin, which can result in a urinary tract infection. "The human skin is like the 'A' side of Velcro and the bacteria is the 'B' side," Schmidt says. "They stick together. "

Public showers are notorious for being contaminated with the plantar wart-causing HPV virus. There are more than 100 different types of this virus, but Schmidt says that they aren't as contagious as other infections. However, they thrive in warm, moist environments -- like the locker room. If you aren't wearing shower shoes and happen to pick up the virus, then it can transfer to your towel when you dry off your feet, Schmidt says. If you are wearing shower shoes, be sure to wrap them up in a plastic bag before packing them up. "HPV is all about direct skin contact," Schmidt says. Typically, the virus finds it's way in through a break in your skin.

And be mindful of your sweat towel. Since you likely have it hanging on the arm of your machine, it can pick up all kinds of germs (including ones that cause the cold and flu), as is the case with any item that tons of people touch. That's why Tierno marks an "x" on one side of his towel, and only lets that side touch the machine.

When it comes time to wash your towels, Tierno says the best sanitizing solution is one part bleach and nine parts water. For this reason, it's best to choose a white towel.

Think about all of the places where you put your gym bag: on the floor, in a locker or cubby, and even on the bench in the locker room. Plus, you shove all of your germ-ridden equipment into it. You need to be aware of what you're bringing back into your home, Tierno says.

In the case of your dirty gym bag, that could be nasty pathogens like strep and MRSA. You can even catch a cold or the flu. Since someone can be infectious with the flu a day before they feel symptoms, there's nothing stopping them from hitting the gym and potentially spreading it to you, says Schmidt.

To defend against illness-causing microbes, Tierno recommends a lightweight, washable gym bag that you can toss in the machine and wash with regular detergent, which he says will eliminate the bulk of your germy problems.

"Most germs are associated with your skin," Schmidt says. We're all covered with microbes, and we shed our skin cells every three to four days -- releasing them and the bugs on them out into the wild. Most of these microbes are friendly, but others are scary pathogens like MRSA (antibiotic-resistant staph) and strep.

You know the smell of your dirty gym socks? That's the wonderful eau de parfum of the waste emitted by all the oil-eating bacteria on them. Schmidt says that using "smell-o-vision" is one of the best ways to ID if germs are present on your gym clothes.

Tierno says that wearing long sleeves and long pants to the gym "catch" all the germs from contaminated equipment. This is good, because it keeps those microbes from coming in contact with your skin. He's done cultures on equipment all over the gym, and discovered all kinds of icky, illness-causing germs (think: staph and the stomach flu, which he says is very easily spread).

Just be sure to toss your clothes in a cloth or plastic bag when you're done: You don't want all those germs you picked up to contaminate the inside of your bag. And don't forget to wash your hands. If there's one piece of advice Tierno was most adamant about, it was hand-washing.

You can also invest in antimicrobial materials, Schmidt says. Lululemon's Silverescent line has silver woven into it, which kills bacteria and, thus, keeps your workout clothes from getting stinky. Another effective antimicrobial is copper. And while polyester doesn't breathe very well, it actually resists oil better than natural fabrics, Schmidt says. This means less bacteria and less lingering stench.

When you wash your workout clothes, your regular soap and water can work fine. But "fine" is usually synonymous with "not so great." Detergents with germicidal ingredients, like the all-nartural Mr. Black Sport Wash, are going to work better because they blast more germs (plus, they help take the stink out of your clothes).

There's a reason why you wouldn't roll around on a sidewalk or the gym floor: It's dirty. And that's where your gym shoes spend most of their time. Schmidt says that a lot of people just toss their shoes in their bag, but essentially they are "toxic waste sites." You're contaminating the inside of your gym bag, along with everything else that's in it, with nasty things like fecal matter.

The trick is to keep them isolated, whether you're headed to or from the gym. Either wrap them up in a plastic bag, or invest in a bag like GymTote or Athleta Warm Up Gym Bag, which both have a separate compartment to stash your sneakers.