Although I've learned my lesson about getting health-related advice from the internet, which is riddled with deliberate and inadvertent misinformation on such matters (even from MD's), and although I am much more cautious about expressing any firm positions (as reflected in the title of this post), it seems to me that raising this question is appropriate at this time.
One post (here) in my health-related blog (which needs updating to state that nothing I've tried has cured my fungal sinus infection, to which large doses of Vit D3 might have contributed) contains excerpts from a biomedical text which indicate that Vit D3 suppresses the immune system. (This post also refers to another post which contains anecdotal evidence by an MD, of a connection between Vit D consumption and immune deficiency.) Considering that Vit D3 has been pushed for quite a while with little resistance, it seems likely that many people have incorporated large doses into their diets, and this might explain some of the news about the current flu outbreak.
Shane Ellison, a.k.a. The People's Chemist, has expressed some very strong anti-Vit D opinions (here), and in a rather salty anti-D rant (here), has stated that the chief Vit D pusher, Dr. John Cannell, founder and Executive Director of the Vitamin D Council, was formerly "a psychiatrist in a hospital for the criminally insane," indicating that he might be a Reesian shrink, and that his current job might be best described as a "behavioral nutritionist," i.e. someone who manipulates people into destroying their health with "nutritional advice." He is all over the internet, pushing large doses of D3.
So, if you've come down with an especially nasty case of the flu, you should consider whether it might have something to do with large doses of Vit D.