How walkers beat colds and flu.
On the other hand, a regular brisk walk can keep winter colds and flu at bay.
Moderate exercise strengthens the body's defences against nose and throat infections such as the common cold, flu and sinusitis, according to Professor Mike Gleeson.
But too much exercise might be as bad as too little for the immune system.
Different levels of exercise significantly increase or decrease the chances of catching an upper-respiratory tract infection (URTI), said Prof Gleeson, from the University of Loughborough.
Prof Gleeson told a conference at the University of Liverpool: "If you have a tendency to be a couch potato then you probably have an average risk of catching an infection - typically 2-3 URTIs per year.
"Research shows that those undertaking regular moderate exercise (eg a daily brisk walk), can reduce their chance of catching a respiratory infection, such as a cold, by up to almost a third.
"Conversely, in periods following prolonged strenuous exercise, the likelihood of an individual becoming ill increases.
He added: "To keep colds at bay, a brisk daily walk should help - it's all about finding a happy medium."