But in some cases, light to moderate activity may actually help you feel better, says Dr. Richard Besser, author of "Tell Me the Truth, Doctor: Easy-to-Understand Answers to Your Most Confusing and Critical Health Questions."
First, Besser says, use the "neck rule": If your symptoms are above the neck -- sneezing, sinus pressure, stuffy nose -- then breaking a sweat is generally considered safe. Listen to your body, and consider the following best (and worst) workout options.
Having a cold may compromise your energy levels, so you may not feel up for intense physical fitness. But even just a 20-minute walk can help you reap the benefits of regular exercise, and it may help improve your cold symptoms, as well.
"If your sinuses are plugged up, walking will stimulate you to take deep breaths and can help open up those passages," says Besser. (Of course, if you discover that walking -- or any type of physical exertion -- makes you feel worse, rather than better, stop and focus on getting rest, instead.) Although there's little research on how exercise can affect the duration of a cold, studies have shown that people who regularly work out tend to get sick less, overall.
As long as jogging is part of your regular routine, there's no reason you need to skip it just because of a mild head cold.
"My patients who are runners all say that running helps them feel better when they're sick," says Dr. Andrea Hulse, a family practitioner (and runner) in Silver Spring, Maryland. "Running is a natural decongestant, and it can help clear your head and feel normal again."
You can scale back the intensity of your normal run, Hulse says, since your body is already working in overdrive to help fight off infection. And the American College on Exercise recommends holding off completely if you're experiencing flu-like or below-the-neck symptoms, like nausea or vomiting.
Best: Qi gong
This type of slow, mindful movement is a cross between martial arts and meditation. It's low-intensity enough for days that you don't feel like breaking a serious sweat, and it has been used for thousands of years to reduce stress and anxiety, improve blood flow, and increase energy. (In Chinese medicine, this is known as regulating and healing the body's "chi," or energy force.)
There is some modern evidence that qi gong has immunity-boosting powers, as well: A 2011 University of Virginia study found that varsity swimmers who did qi gong at least once a week came down with 70% fewer respiratory infections that their teammates who practiced it less often.
Worst: Endurance running
Training for a marathon? Skip this weekend's long run if you're sick -- even if you're already getting over, or just feel yourself coming down with, a cold.
"In general, regular exercise stimulates the immune system and helps keep us healthy," says Hulse. "But too much regular exercise at a high intensity can have the opposite effect," she adds.
While no studies have looked at the effects of endurance running while already sick, Hulse says, its overall strain on the immune system is well documented: A 2007 study published in the Journal of Applied Sciences, for example, reported that immune function may be compromised for up to 24 hours after prolonged, continuous exercise (1.5 hours or longer).
The body releases the stress hormone cortisol while it's fighting infections like the common cold, and research suggests that stress-relieving techniques -- such as yoga and breathing exercises -- may help boost immunity. Plus, says Besser, gentle stretching may help relieve aches and pains related to colds and sinus infections.
Choose a slower style of practice, like Hatha or Iyengar yoga, if you're worried about overdoing it with vigorous sun salutations. Or focus on restorative postures, like Child's Pose and Legs Up the Wall, at home.
And don't forget to say "om": A Swedish study found that humming is a good way to open up clogged sinus passages.
Worst: Machines at the gym
In addition to how you exercise when you're sick, it's also important to consider where you exercise: "If your workout involves going to the gym and being in close contact with other people, you need to ask yourself if you'd want someone else with your symptoms doing the same thing," says Besser.
"If you would not like the person next to you on the treadmill or who finishes before you on the elliptical to be sneezing and coughing and wiping their nose, than do your fellow gym mates a favor and do a lighter workout at home, instead."
Germs can spread easily on machines and in the locker room, he adds, so it's best to stay away while you're contagious.
Taking a Zumba or cardio dance class -- or even just rocking out to your favorite tunes while you clean the house -- can serve as a stress-reduction technique. In fact, one study found that people who just listened to 50 minutes of dance music had less cortisol and more cold-fighting antibodies -- a sure boost to their immune systems.
Dance classes tend to be low impact, so you can break a sweat without putting too much stress on your joints (or aggravating a cold-related sinus headache). You can go at your own pace, as well: Take it easy on days you're not feeling 100%, and try to just enjoy the party.
Worst: Lifting weights
Your strength and performance will likely be diminished while you're battling a cold, says Besser -- especially if you've missed out on quality sleep -- putting you at increased risk for injury while trying to lift heavy equipment. Plus, the muscle strain required to lift weights can cause sinus pressure and headaches to feel even worse, he adds.
Still don't want to skip a strength workout? Do it at home, where you won't be spreading germs and sharing your sickness with other weight lifters, and give yourself a break by using lighter dumbbells than usual. (Increase your reps, not the weight, if you need more of a challenge, says Hulse.)
Best or worst: Swimming and biking
Like walking and jogging, other forms of moderate cardio can help clear congestion and boost energy levels, says Hulse -- but they won't work for everyone.
"It's really a matter of personal preference, what type of symptoms you have, and what your normal routine is like," she explains.
Swimming, for example, can feel quite refreshing, and may help open up airways. (For people who suffer from allergies, it can also help by washing away pollen and dust.) But some people may find it difficult to breathe while congested, or may be irritated by chlorinated waters. Biking can also be a nice, moderate exercise, but may dry out nasal passages and increase symptoms like sore throat and runny nose.
Worst: Team sports
Just like using the treadmill or weight machines at the gym, playing sports that involve physical contact can encourage the spread of illness.
"If you're a pro athlete, then your coaches and teammates may expect you to be out there no matter what," says Besser. "But in something like a friendly neighborhood basketball league, they're going to thank you for sitting one out."
Cold and flu viruses spread through droplets, like tears and saliva -- but also through hand-to-hand contact, he adds.
"If you wipe your nose and then you pass the ball, you've just passed those germs on, as well."
A 2011 study from the Centers for Disease Control and Prevention found that the sports teams are at high risk for outbreaks of the stomach flu norovirus among members.
Worst: Anything outdoors in the cold
Working out in freezing temps may be detrimental to some people battling cold symptoms, but not for the reason you may think. Contrary to popular belief, cold weather in itself will not lower immunity or cause you to get sick -- not even if you go outside without a coat or you sweat so much your hair gets wet.
What can happen, however, is that cold, dry air can restrict or irritate airways -- triggering a runny nose, coughing, or asthma-like symptoms, says Hulse. If you find that you are sensitive to these conditions, winter activities like skiing, snowboarding, or snowshoeing might be even more difficult when you have a cold.
Plus: What about allergies?
Sometimes, what people think of as recurring cold symptoms -- sneezing, headaches, nasal congestion -- are actually allergies in disguise.
"If you find that you are seeing those symptoms come on at the same time each year, you might want to ask your doctor about getting tested," says Besser.
Allergies to pollen and ragweed can make outdoor exercise difficult in the spring and fall, he adds, while allergies to dust, mold or harsh cleaners can be triggered by workouts at the gym or in other enclosed spaces. If you can pinpoint the cause of your symptoms, an antihistamine or other treatment can likely help you get back to your normal life -- and your normal workout routine.