Couch potatoes are nearly twice as likely to catch a cold, and a third likelier to suffer bad symptoms of a cold, compared with counterparts who keep fit, American researchers reported.
They tracked 1,002 adults in Wisconsin aged 18-85 for 12 weeks in the autumn and winter of 2008, monitoring them for respiratory illness and weight and quizzing them about diet, lifestyle and aerobic exercise.
People who described themselves as fit or who exercised up to five days a week or more, had between 4.4 and 4.9 "cold" days on average.
For those who fell in the middle category of fitness, and exercised between one and four days a week, this was 4.9-5.5 days.
But among counterparts who said they were of low fitness and who exercised only one day a week or less, the tally was between 8.2 and 8.6 days.
Good fitness also caused the severity of cold symptoms to fall by between 31 and 41 percent lower compared with the most sedentary lifestyle.
Bouts of exercise unleash a temporary rise in immune defences, helping to boost preparedness against viral intruders, the study suggested.
It cited figures that the average adult in the US can expect to have a cold two to four times a year, and children between half a dozen and 10 colds a year. The cost to the US economy is put at around 40 billion dollars annually.
The paper, published in the British Journal of Sports Medicine, is headed by David Nieman, director of the Human Performance Laboratory at the Appalachian State University in North Carolina.