An area of research that interests me is the ongoing search for an answer to the question: why is seasonal flu seasonal? Nobody knows. That's what makes it interesting. One promising line of inquiry has to do with seasonal variation in levels of sunlight. We know that decreased sunlight is associated with seasonal affective disorder. The most logical implication is that it has something to do with the way exposure to sunlight influences production of hormones (vitamin D is one; there may be others that play a role and which are also influenced by exposure to light). It seems reasonable that vitamin D (and whatever else) may influence immunity in ways which go beyond that which is presently understood. As has been said, vitamin D is not easily absorbed from supplement, and vitamin D deficiency is a widespread problem. Therefore, being particularly at risk from influenza, I try to get twenty minutes or so of direct sun every day during flu season, whenever it's possible to do so without freezing my buns off (I live in California, where such opportunities are not rare). I realize that in doing this, I am accepting the possible risks that are also associated with exposure to sunlight (specifically, UV). I find this tradeoff acceptable. It's the same logic that persuades me to get a flu shot every year.