P.S. I work in a shipyard and don't do much "handwashing" there and am around people with sniffles and coughs all winter.
Of course, I can't "prove it." I don't have a medical lab and am not equipped to do a study of thousands of cases. 100% of a dozen or so cases is sufficient indication to me that the results I experience are such as to indicate that it is worthwhile to proselytize it to people I care about. If you and I were neighbors I would happily wager it with you. You might try researching it. I was skeptical when I heard about it and looked it up on the net. I tried it first because it is cheap and D-3 is certainly not toxic. I suppose someone somewhere must be allergic to it, there are folks who cannot tolerate sunshine. I've read about such but never run into one.
Believe me, I’m glad you have stayed “flu free” for so long. You don’t want it. However, there is no cure or substance that will rid you of the virus once it enters your system. If you have managed to avoid it, there could be other factors that have kept you from acquiring the virus. However, Vitamin D-3 is no cure for the flu, nor is it an immunogen.