Specifically, it was Joe Friel's "neck check" to help decide to train while sick or not:
If you have above-the-neck symptoms, such as a runny nose or scratchy throat, start your workout, but reduce the intensity and duration. you may begin to feel better once you're warmend up, but if not, stop. If the symptoms are below the neck -- such as a sore throat, chest cold, chills, coughing up matter, achy muscles, or a fever -- don't even start. These are often the symptoms of a virus. Exercising will make it worse.Now, Gina Kolata writes in the the NYTimes Personal Best section that studies have shown that exercise actually helps you down the road to recovery. Based on the results, this winter I'll keep pushing through when feeling ill (within reason).
Back to the proof: the decade-old studies asked two questions, 1) does a cold affect your ability to exercise, and 2) Does exercising when you have a cold affect your symptoms and recovery time?
Regarding one's ability to exercise:
At the start of the study, the investigators tested all of the subjects, assessing their lung functions and exercise capacity. Then a cold virus was dropped into the noses of 45 of the subjects, and all caught head colds. Two days later, when their cold symptoms were at their worst, the subjects exercised by running on treadmills at moderate and intense levels. The researchers reported that having a cold had no effect on either lung function or exercise capacity.
“I was surprised their lung function wasn’t impaired,” Dr. Kaminsky said. “I was surprised their overall exercise performance wasn’t impaired, even though they were reporting feeling fatigued.”
And in response to how exercise affects recovery time:
The investigators found no difference in symptoms between the group that exercised and the one that rested. And there was no difference in the time it took to recover from the colds. But when the exercisers assessed their symptoms, Dr. Kaminsky said, “people said they felt O.K. and, in some cases, they actually felt better.”
So, if you feel a cold coming on, do the neck check and keep going if it's all in your head. As Kolata points out at the end of her article, "too often taking time off because of a cold is the start of falling away from the program entirely."