Think of your body like a castle, complete with its own army—your immune system—to defend it.

When germs try to invade your body, your immune system recognizes them as foreign and launches a defense. First, your skin tries to prevent germs from getting inside your body. Your mucous membranes try to trap and fight off the germs. And your body makes white blood cells to repel the invaders.

After the skirmish, your immune system remembers the foreign substance. If it ever attacks again, your immune system will remember how it fought back the first time—even if it didn't completely succeed. That way, most of the time you won't get sick again. You'll have immunity.

It's important to help keep your immune system healthy so it can continue to fight off invaders. You can do that in several ways.

Practice good hygiene. Wash your hands often to prevent the spread of germs. Wash produce before eating it. And be sure to thoroughly clean your dishes and utensils to prevent the growth of bacteria.

Reduce stress. Stress can weaken your immune system. Find ways to manage stress, such as practicing deep breathing exercises or listening to music.

Get moving. Exercise can help your body produce immune cells that fight off disease.

Make healthy sleep a priority. Poor sleep can weaken your immune system. Adults should try to get seven to nine hours of sleep a night. Children need as much as 8 to 14 hours of sleep, depending on their age.

Eat well. Go for foods that contain the following nutrients, which help support the immune system:

• Protein. It's found in both animal and plant-based foods such as fish, beef, chicken, eggs, beans, lentils, and nuts and seeds.

• Vitamin A. It is found in foods like sweet potatoes, carrots, broccoli, spinach and fortified foods.

• Vitamin C. Citrus fruits, tomato juice and food fortified with vitamin C are all good sources.

• Vitamin E. You'll find this vitamin in fortified cereals, sunflower seeds, vegetable oils and peanut butter.

• Zinc. Good sources include lean meat, poultry, fish, whole-grain products, beans, seeds and nuts.

• Probiotics. These good bacteria are present in cultured dairy products, like yogurt, and in fermented foods, like kimchi.

Sources: Academy of Nutrition and Dietetics; American Heart Association; National Institutes of Health