Two years ago I read about a study that indicated a corelation between vitamin d deficiency and colds or flu.
So I started taking calcium and vitamin d supplements, and for the first time, I did not get a cold or flu during the winter. This worked again last winter too.
I was exposed to the flu, and the doctor had me take an antiviral during one of those winters.
The same with ZINC; I have been taking 50 mg. a day for over 12 years and have only been sick once.
I take 800-1200 I.U. Vitamin A a day.