Shape up once your sniffles subside with this post-sickness plan to get you moving again.

A winter cold, flu, or virus can throw a serious wrench in your normally fit routine, leaving you bedridden and (eventually) craving a good sweat. (Next time, try these tips to fight cold and flu germs the right way.)

But how do you get back on the bandwagon after an illness passes? How big of a setback did you really take? And how much exercise is too much when you're first getting back at it? We touched base with Michele Olson, PhD, a professor of exercise science at Auburn University Montgomery to find out.

Things might not be as bad as you think: If you're flat on your back for a week-assuming you stick to a regular gym routine when you're healthy-you'll lose about 30 percent of your fitness, especially your cardio output, says Olson. While this is a bummer, with two to three weeks of training-using the right bounce-back strategy-you should be close to your normal physical fitness again, she says.

So how can you tell if you're OK to hit the pavement? First and foremost, make sure you haven't had a fever for at least 48 hours, says Olson, who adds that you should also have a few good night's sleep under your belt, and no longer have any aches and pains.

"If you're running a fever, you should not work out," Olson notes. "The energy needed by your immune system to fight off bacterial infections will be compromised if you exercise." And this means you'll invite lingering symptoms to worsen-which could predispose you to more intense issues like mononucleosis or pneumonia, she says. (Not fun.)

So if you truly think you're in the clear, it's important to ease back into your regular routine. "When you've had an infection, the increased work of your immune system is taxing on the body," says Olson. Overwhelm an already over-worked bod and you'll wind up right back in the sack.

As for where to start, Olson suggests light cardio then resistance training. "It's important to make sure your oxygen delivery system is intact so that when you do resistance training, your muscles will get the oxygen," she says. But if you're a yogi (and your body is familiar with the practice), you should be OK returning to the studio with a light class, since the exercise is less demanding and often moves at a moderate cardio pace, she adds. (Try these 5 Yoga Moves to Beat the Flu!)

The bottom line: Don't naively assume you can go back to 100 percent right away. "Do about 70 percent of what you were doing," suggests Olson. Reducing your weights and cardio output by 30 percent for a few days will make up for the loss in fitness while you were sick. So build back slowly, even if you're tempted to push harder. Don't worry, eventually, two miles won't feel like 10 anymore.