When I was young and otherwise healthy I would get 2 or 3 terrible colds (flu) per yr. Fast forward with aging and a severe and chronic toxic condition, I might get 1 cold per yr and then only briefly. I take nutritional supplements by the handfuls but one that seems the absolute essential immune support is Zinc. Zinc is one mineral that is terribly lacking in the western diet AND seems to be the cornerstone of the immune system, so it is almost necessary to supplement in every person to prevent such as the flu. A healthy Thymus is also critical. Some researchers believe that the power of Zinc is the strengthening of the Thymus.