Dropping temperatures can mean the rise of influenza. According to the CDC, flu activity starts to pick up in October, peaking between December and February. How can you boost your immune system so that you can avoid being sidelined by the flu and get the most out of fall? Here are some of the best immune-boosting foods and best practices to help you steer clear of the flu. Preventing the Flu The best way to avoid contracting the flu? Get a flu shot, The Centers for Disease Control and Prevention says. The immune system is the body's first line of defense against flu viruses, and the flu vaccine will help your body defend against specific influenza viruses. The CDC also recommends a number of preventive actions, such as limiting contact with sick people, disinfecting surfaces and washing your hands often. Eating a diet rich in nutrients, such as vitamins A, C and E, can also help your body fight off illness. The U.S. Food and Drug Administration says that one of the best ways to give your body the nutrients it needs is by eating a variety of nourishing foods. Some people may benefit from a vitamin supplement to meet their needs. You should always talk to your doctor before taking any supplements. You should also talk to your doctor if you think you have the flu, so keep an eye out for the flu's telltale signs and symptoms.