http://www.cleveland.com/healthfit/index.ssf/2010/04/vitamins_minerals_fill_nutriti.html

1) “There is an epidemic of vitamin D insufficiency in the United States, the public health impact of this observation could be great.”
2) “The occurrence of the common cold and influenza shows clear seasonality. The cold and influenza season corresponds to the season of vitamin D insufficiency.”
3) “The lack of vitamin D during the winter may be a ‘seasonal stimulus’ to the infectivity of the influenza virus.”
4) “Vitamin D is produced in the skin when sunlight is absorbed. Thus, vitamin D levels, or serum 25-hydroxyvitamin D (25-OHD), fluctuate seasonally.”
5) Vitamin D has important functions in the immune system, specifically the innate immune system.
6) Over a 3-year period, taking 800 IU of vitamin D3 reduced the incidence of colds and flus by 70%. Taking 2,000 IU of vitamin D3 reduced the incidence of colds and flus to nearly zero (only one case out of 104 users).
7) “Vitamin D supplementation, particularly at higher doses, may protect against the ‘typical’ winter cold and influenza.”
28) “The physiological basis of the protective effect of vitamin D lies in its ability to stimulate innate immunity and to moderate inflammation.”
9) “These reports provide a rationale for vitamin D supplementation in the prevention of colds and influenza.”
10) Only vitamin D3 is bioactive; vitamin D2 (ergocalciferol) “is not vitamin D but a less potent vitamin D analogue that plays no role in normal human physiology.”
11) “Physiological doses [800 – 2,000 IU / day] of vitamin D prevent many viral respiratory infections.”
12) “It is also reasonable to postulate that pharmacological doses of vitamin D may be effective adjuvants in a breathtakingly large number of life-threatening infections.”
[my emphasis added]

Though we may not be able to prevent an outbreak, it is best to find ways to improve immune function. Our bodies will better be able fight off the virus.