It is reported as saying that moderate exercise is good whereas chronic endurance exercise is a problem. It is not clear where strength training would fit in to this analysis:
The major players in this immune regulation are immune cells called Natural Killer (NK) cells which are important weapons in the fight against viral infections. NK cells recognise viral-infected cells as foreign invaders and force them to commit suicide." During moderate exercise the activity of NK cells is enhanced, whereas stressful endurance activities such as marathons can turn down NK cell activity. These changes are tightly regulated by stress hormones and other immune cells," explained Professor Gleeson.
There is a clear take-home message from our current understanding of the link between exercise and immune function. "Moderate exercise has a positive effect on the immune system. So to keep colds at bay, a brisk daily walk should help - it's all about finding a happy medium," said Professor Gleeson.
The research is reported here: Couch potato or elite athlete? A happy medium keeps colds at bay!