For years I have found it very effective to fight off an on coming cold or flu with an immune boost supplement of 50 mg chelated zinc. Always best to take with food in your stomach or you may experience some stomach upset. Personally I take the zinc as soon as I feel some thing coming on and continue to take it once a day until the symptoms are clear. I have literally watched my self fight off symptoms before they can get a hold of my system time after time. As well the duration of the cold or flu also appears to be lessened in many cases.