Taking regular, moderate exercise cuts the risk of colds, research suggests.
A trial of the effect of exercise in post-menopausal women found up to a three-fold reduced risk of cold in those who did more physical activity.
And the ability to ward off colds grew stronger over time, the study in the American Journal of Medicine found.
The researchers from Seattle advised 30 to 45 minutes of moderate daily exercise as overdoing it may weaken the immune system, they said.
The study was set up to look at the effect of exercise on breast cancer risk.
Half of 115 previously sedentary, overweight women who had gone through the menopause were asked to do moderate aerobic exercise for 45 minutes five times a week.
The other half attended a weekly stretching class.
Most of the women in the exercise group chose to do brisk walking either in the gym or at home.
Over the year-long study, those in the non-exercise group had twice as many colds as those who increased their physical activity.
The effect was seen despite the fact that the women in the exercise group only managed on average 30 minutes of exercise five days a week.
The improved immune function was greatest in the last three months of the study, where those who didn't do exercise had a three-fold increased risk of colds.
Women who exercised also had lost a significant amount of weight and total body fat.
The researchers said previous research had shown exercise could increase levels of leukocytes, a type of white blood cell that helps fight infection.
Risk of upper respiratory infection, however, was not different between the two groups but more women in the non-exercise group had a flu jab which could have affected the results.
Study leader Dr Cornelia Ulrich, Fred Hutchinson Cancer Research Center, said: "The enhanced immunity was strongest in the final quarter of the year-long exercise intervention."
"This suggests that when it comes to preventing colds, it's really important to stick with exercise long term," she added.
"In everybody this level of aerobic exercise has been recommended for a number of health outcomes. The women also lost a bit of weight and fresh air could go some way to helping strengthen the immune system."
"It's important to bear in mind that this is the first study to show this and ideally we would like a much larger trial."
Professor Ron Eccles, director of the Common Cold Centre at the University of Cardiff said: "I've always supported exercise as a way of improving health so I was pleased to see this."
He added the researchers conclusion that exercise increases levels of leucocytes, boosting the immune systems ability to fight off infection was perfectly reasonable and backed by bits of evidence.
But said: "One wants to make sure that exercise was the only difference between the two groups. By doing exercise there might be a change in psychology or they may sleep better, which is also good for the immune system.
"You'd also have to make sure the two groups had the same exposure to colds as women who stay at home get more colds because they have more exposure to children."