When is a cold contagious?

A cold is most contagious within the first 2-3 days. It's contagious throughout the entire affliction, but that's just the high point. After 3 days, you're so barely contagious that you'd have to sneeze into the mouth of a person without an immune system to really get someone sick (exaggerating a little), but the point is, a cold is still contagious at any stage.