Without vitamin D, you're a sitting duck
What this study reveals is that without sufficient levels of vitamin D circulating in your blood, you're a ripe, juicy target for influenza (H1N1 or otherwise). If you lack vitamin D, your immune system can't "activate" to do its job. That's why people who are deficient in vitamin D so frequently get winter colds.
But people who are high in vitamin D have the nutritional power to activate their immune system so that it can respond to invading pathogens. Crucially, vitamin D also manages to balance immune response and prevent inflammation -- the leading cause of death in the 1918 influenza pandemic.
So not only does vitamin D protect you from the initial infection; it also prevents your body from over-reacting and killing you with inflammation (which typically gets expressed as bacterial pneumonia, an infection of the lungs).
