There have been multiple studies that have correlated low vitamin D levels with increased infection and we now understand that vitamin D modulates both our innate and adaptive immune responses. This also ties into our understanding of the role seasons have in our fluctuating immune susceptibility. The cold weather we experience in winter doesn’t inherently cause a surge in influenza, but rather, the shorter days, decreased sunlight exposure leads to one thing – significantly less vitamin D – a biological breeding ground for illness susceptibility.