Research suggests that L-theanine, found in black, green, oolong, rooibos and pekoe teas may enhance the disease-fighting capabilities of the immune system. The herbal teas 
strengthen the T-cells of the body’s immune system, allowing them to better defend against invading bacteria.
Just drink 2 to 4 cups of tea per day for a potent brew of amino acids and antioxidants. For an added benefit, the dietary phenols present in herbal teas also inhibit the growth of harmful bacteria in the stomach. I already drank six cups of different herbal teas today.