I've heard the same thing, but don't know for sure. I tried a google search but didn't come up with anything. What I gather, though, is that you are contagious before you start showing symptoms (it takes a few days for a cold to develop fully), but are also contagious after you start showing symptoms, at which point you've already infected everyone you've come into contact with. I guess this is why everybody starts showing visible symptoms at work at the same time.