how common cold is caused

Age. Children younger than six are at greatest risk of colds, especially if they spend time in child-care settings.
Weakened immune system. Having a chronic illness or otherwise weakened immune system increases your risk.

Time of year. Both children and adults are more susceptible to colds in fall and winter, but you can get a cold any time.
Smoking. You’re more likely to catch a cold and to have more severe colds if you smoke.