Our bodies are under constant attack from infection-causing bacteria and viruses. A healthy immune system—well-armed with white blood cells, antibodies, proteins and other substances—can destroy or inactivate those foreign invaders, and might also spot and eliminate newly formed cancer cells from within your body.

But a number of forces, including advancing age, can weaken those defenses, increasing your risk of infection and possibly cancer, and slowing your recovery from illness. Fortunately, there are steps you can take to fortify your immune system, including eating and exercising wisely, reducing stress, and practicing other healthy behaviors. Those measures offer protection even in older adults, challenging the long-held notion that immunity inevitably declines with age. Here's what you can do to toughen your body's armor against disease.

This article first appeared in the December 2008 issue of Consumer Reports on Health.