Polyphenols, potent plant antioxidants, are what’s believed to give green tea its immune-boosting effects. One laboratory study suggested that a particular type of polyphenols called catechins may kill influenza viruses. To maximize benefits and minimize bitterness, use just-below-boiling water and steep green tea no more than a minute or two. A little lemon and honey can also help blunt the bitterness. But don’t add milk, because the proteins will bind to the polyphenols, making them ineffective.