Ditto, Vit D deficiency is rampant and my level was quite low.
I take that and a probiotic and avoid sugar, especially in flu season.
I up my C and Niacin in flu season. I also use hand sanitizer. High doses of zinc if I feel any symptoms. Fingers crossed since the flu has skipped me for years and years and years.
In addition, it’s important to take a good multivitamin and Omega 3 fish oil. I work 2 jobs and am constantly on the go, so unfortunately I don’t have time to adhere to the gubmint’s socialistic food pyramid. Several nights a week, my meal consists of a TV dinner, and I hate eating fruit outright. So, it’s vitamin supplements for me.