A study showing that a substance in Fruits and Vegetables known as ''Quercetin'' enhances the bodies ability to fight Stress and the Flu. Quercetin is found in Onions, Apples, Berries, Brocolli, Tea and Red Wine etc.
I'm sure there's alot more health benefits besides just preventing the Flu like Cancer and Aging but here's the article for those interested.
Science Daily-Substance Found In Fruits And Vegetables Reduces Likelihood Of The Flu