Surely you've heard the old saying, "an apple a day keeps the doctor away." While a daily apple is a better strategy than nothing, it'll actually take a much stronger plan to really boost your immune system. Almost every year, we hear that this cold and flu season is predicted to be a doozy, and we're cautioned to be proactive to fight against germs. The best weapons in germ warfare are adequate sleep, regular exercise and appropriate hand washing. But don't stop there: Give your immune system an all-natural "booster shot" by eating more of these readily-available, budget-friendly foods.