One of the best ways to ward off a pesky cold is to strengthen your immune system against bacteria, viruses, and other invaders. The absolute best way to boost your immune system is to eat a diet rich in fruits and vegetables and to get plenty of regular exercise. It's true — exercise has a proven, powerful effect on immunity. A study revealed that when unfit, sedentary people started briskly walking 45 minutes a day, five days a week, they caught fewer colds over a one–year period. That's because each and every time you moderately exercise, the immune system — your body's first line of defense — starts functioning at a higher level and remains elevated for about three hours.