h2g2 The Hitchhiker's Guide to the Galaxy: Earth Edition

Getting regular exercise helps me ward off most colds. That, and washing hands frequently. Exercise just keeps you in better shape, so that having a minor virus won't put you out of work.

It's also worth pointing out that exercise can be helpful even while you're sick (assuming you have the strength.) Exercise raises your body temperature which, like a fever, helps to destroy the microbes that have invaded your body. Also, the act of sweating (and then drinking a lot of water to replenish the body's fluids) is a cleansing process that can help to eliminate toxins left in your body by the microscopic invaders.

-J.P.