There are important substances in the fruits and vegetables that we consume that influence the immune system. In an article entitled "Effect of Flavonoids on Upper Respiratory Tract Infections and Immune Function: A Systematic Review," investigators found that those taking flavonoid supplements (dosage ranging from 0.2 to 1.2 grams a day) had a third fewer incidence of upper respiratory infections. They conclude, "These findings suggest that flavonoids are a viable supplement to decrease URTI incidence in an otherwise healthy population." Flavonoids are easily found in the foods we eat. For more information on which foods contain flavonoids, see