"Shoot, I'm getting sick. Can I still train?"
If it is a head cold, then yes. Keep moving, but be ready to take it down a notch. There have actually been studies done that show an increase in white blood cell production with moderate exercise. The goal is to aim for moderate. Too hard a work out and you may end up exhausted and the cold can take over.
Chest cold, try it but be ready to head to the couch. We all know how important it is to be able to breathe!! Our muscles need the oxygen and we don’t want the exhaustion component that goes along with oxygen shortage.
If it is stomach, then generally no. Take a few days off. It is difficult for the body to perform at the level we expect when fuel and hydration may be jeopardized by stomach / intestinal misbehavior.
Rule of thumb.
Neck and above, go for it – in moderation. Below the neck, rest, preferably with a good book.