It has long been recognized that patients with tuberculosis do better when treated with vitamin D or exposed to sunlight. It was recently recognized that the immune cell known as the macrophage needs vitamin D in order to produce a peptide which is responsible for killing infectious agents such as tuberculosis. It has been speculated that one of the reasons that influenza occurs in the winter time in tepid climates is because the sun is unable to produce vitamin D, and the resulting vitamin D insufficiency may promote and enhance the infectivity of the influenza virus.
