Hello my brethren!
I have been reading some more information regarding treatment, preventatives, etc of the flu for the booklet I am (honestly) almost done writing.
I had known for many years that vitamin D was a very necessary nutrient. I found another article regarding such that I think is very worth your time to consider reading:
This article talks about vitamin D as preventative for the flu. It is an anecdotal report of a psychiatrist who worked with the criminally insane. In his experience on spring, patients in all wards EXCEPT HIS got the flu during an outbreak. His ward was not isolated--nurses worked in all wards. After reading a study, he realized that his ward was the only one where the patients were getting vitamin D.
This article brings up many other interesting reports about the health of people in various latitudes, outbreaks of illnesses at the same latitudes, why the elderly of some countries die more often in winter, while the elderly of others do not.
Another significant fact about vitamin D is that can decrease the expression of inflammatory cytokines. The over preoduction of cytokines is what leads to a "cytokine storm"--that is what killed most of the healthy young adults who got the flu in 1918. http://www.ncbi.nlm.nih.gov/sites/entrez?cmd=Retrieve&db=pubmed&list_uids=16959053
One interesting point in another article I read is that the latitude where one can get USABLE vitamin D during the winter is much further south than I would have expected. In our latitude, just south of the arctic circle, we are getting no vitamin D from the sun, even if we stand outside for hours in the winter.
For our family, we are drastically upping our vitamin D intake. The official "upper limit" of vitamin D is 2,000IU. However, most researchers consider that too low, and toxicity is not seen, apparently, until one reaches over 40,000IU.
I was of the mind that drinking milk would be sufficient, however, milk only has 100 IU.
With our society being afraid of sun damage, as well as kids not drinking as much milk (and some US children showing up with RICKETS!), it as assumed that much of the country has a vitamin D deficiency.
Coupling the positive effects that it seems to have on general respiratory health, the possibility that it prevents flu infection, and its ability to moderate cytokines, I would recommend considering adding vitamin D to the diet. The vitamin D you are looking for is D3...D2 requires sunlight to produce D3.