If you are a top-trained competition cyclist ... or were one within the last decade or so ... you maybe should NOT ride at anything above a leisurely pace with a viral infection. I still recall when one of our olympic and world champion x-country skiers died on a local skiing race a few years after retiring from the world-wide racing game. He'd had the flu a week before, was breathing clear again, but the remnants of the viral infection were enough to cause a heart attack when he got his pulse up to racing speed. It's actually not as dangerous for us wimps who've never been in that kind of shape that brings the heart up to the max of what it can handle. Also, being wimps, when our heart rate starts getting up there and we're breathing hard, we back off. But if you've ever been in really top shape, and if you've learned not to back off from exertion to the level of pain, then do be cautious when you have or recently have had a cold or the flu.