Running is one of the most popular forms of exercise in the United States, and it's easy to see why. The only equipment you really need is a good pair of running shoes, and you can go for a jog just about anywhere. Plus, running is meditative, burns calories, builds your endurance and tones your body.

Many runners find the activity addictive. You've probably heard of the so-called "runner's high" that joggers covet. Running releases endorphins into the brain, and endorphins are responsible for mood changes. The harder you run, the more endorphins released, and the giddier your "high." Some studies show that this endorphin rush allows people to continue running even when injured.

And that brings us to the point of this article -- why people continue their running regimens even when they're sick. Most of us curl up in bed with some chicken soup and the remote control when we're under the weather. But runners often hit the treadmill or the road. Are they helping or hurting themselves by jogging with a cold? How do you know when it's OK to run and when it's better to take off the trainers and climb in bed?

The best way to decide whether you should exercise is to employ what doctors call the "neck check." Feel free to go running if your cold is "above the neck." Above-the-neck symptoms include:

  • Runny nose
  • Nasal congestion
  • Sneezing
  • Sore throat

Doctors advise against proceeding with your exercise regimen if your symptoms are "below the neck," however. These symptoms include:

  • Chest congestion
  • Hacking cough
  • Nausea or upset stomach
  • High fever
  • Fatigue
  • Body and muscle aches

Some people think that running in cold temperatures will actually make them sick. But this isn't really true. You can't freeze your lungs or windpipe. Your body heats the air you breathe. When the air is particularly cold, you may feel a burning in your chest as you inhale. If that's the case, try covering your mouth with a scarf or wearing a ski mask. That'll help heat up the air before you inhale it.

Keep in mind, though, if you have an infection in your chest or throat, running outside can indeed make things worse (according to the below-the-neck rule). But if you have a simple head cold, it should be fine to take that run, even if it's cold outside. The adrenaline running provides can even help clear up a stuffy head.

Next, we'll talk a bit about the positive effects of exercise on your body and understanding your limits.