Spending the day in a stuffy room with anyone who's under the weather raises your risk of catching a bug. Letting a little fresh air circulate keeps airborne viral particles on the move, making them harder to pick up, says Dr. Phillips.

Better yet, go for a brisk walk! Exercise can boost your immune function, which may keep you from getting sick. A 2009 study found that people who exercised 45 minutes or more at a time, about four times a week, took 25% to 50% less time off from work during cold and flu season compared with couch potatoes.