Exhausting exercise weakens the immune system, but moderate workouts have the opposite effect. They temporarily strengthen your defenses by boosting the circulation of white blood cells throughout the body, which allows them to detect invading and infected cells earlier. Repeated often enough, these short-term boosts appear to yield lasting benefits. Exercise may also enhance immunity by flushing out bacteria from the lungs and ridding the body of carcinogens through increased output of urine and sweat.

The benefits of regular exercise extend to previously sedentary seniors, according to a new study from the University of Illinois at Urbana-Champaign.

Researchers randomly assigned 150 physically inactive men and women, ages 63 to 82, to a group that did 10 months of moderate aerobic exercise or to one that did balance and flexibility training. After completing the program, the aerobic exercisers experienced longer-lasting protection from the flu vaccine.

The recommended regimen to energize your immune system is the same as for general health: 30 to 60 minutes of moderate aerobic activity—such as brisk walking—five days a week or more.

The workout should be challenging but not draining. "If you can carry on a conversation while exercising but you're slightly out of breath, that's moderate," says Riva Rahl, M.D., medical director of the Cooper Wellness Program in Dallas. "Adding resistance-training is the fat-free icing on the cake. You get a slightly increased immune function effect from doing both."