People who have demanding activity programmes must balance their enthusiasm with good sense. The first step is to find what amount of activity feels best for you.

It's also important to allow time between hard training sessions. A "hard day — easy day" routine is a good one to follow.

Finally, beware of "overtraining." Cut back or take a day or two off if you experience any of the following symptoms: tiredness that lingers after your session, faster pulse rate in a morning, difficulty in sleeping, or a build up of tiredness.

If you have a cold, light activity shouldn't be a problem. But let your body decide! If you feel well enough to exercise, do it at a more low-key pace than usual.
If you have an upper-respiratory infection (symptoms include chest congestion, coughing, and difficulty in breathing): Don't exercise! When you feel better and symptoms are disappearing, ease back into your normal routine.

Research to date shows that moderate exercise may actually help the immune response: by boosting the body's ability to fight off colds and other illnesses. On the other hand, intense training programs may suppress the immune system and increase the risk of infections. This doesn't mean that if you follow a vigorous activity program you should reduce it. It means being sensible about it.