Spectral Density reviewed a variety of immunological changes associated with diminished UV radiation exposure in experimental settings, and noted that in granulocyte and monocyte function were reduced during periods of short light exposure [33] . UV radiation also influences the production of 1,25-(OH) 2 -vitamin D, which has important immunomodulatory functions [34] . Namely, enhanced maturation of macrophages, macrophage secretion of bactericidal substances such as lysozomal enzyme phosphatase and hydrogen peroxide [33, 35] , and secretion of antimicrobial peptides (including cathelicidins and defensins) by both immune cells, and respiratory tract epithelium [36] . Vitamin D deficiency is associated with a marked increase in the risk of pneumonia [37] , and most human vitamin D is acquired via sun exposure. Thus, extended periods of low UV light could result in an increased susceptibility to S. pneumoniae infection resulting from a lack of vitamin D production, though the week-to-week fluctuations in risk described in this paper may be too rapid to represent a vitamin D effect.