Tim Cable, professor of Exercise Physiology at Liverpool John Moores University, replies: The immune system can be both positively and negatively affected by exercise. Generally, those individuals that are training moderately (three times per week, 60 minutes per session at 70% maximum HR) should suffer fewer colds and flu because this volume of exercise increases the activity of the immune system. The cells of the immune system become more 'switched on' and increase the surveillance of invading bacteria/viruses.
In comparison, people performing high training volumes and competing at high intensities are likely to experience a greater incidence of upper respiratory tract infection, as certain parts of the immune system are 'switched off'. This is because high volumes of training (eg marathon training or a sudden increase in training volume) increase the levels of stress hormones such as adrenaline and cortisol that turn off the white blood cells that search and destroy unwanted invaders.
The recommendation is that when commencing training or increasing training volume, do it gradually (to reduce the risk of elevating stress hormones). If training hard, take sports drinks during and at the end of the session, as glucose appears to reverse some of the adverse changes in immune function during and after recovery from hard exercise.