There is... we get more colds and flu during winter because our Vit. D levels drop. Vit D. helps your immune system to fight off infection and when we aren't spending lots of time out in the sun with our skin exposed we are unable to create this important hormone-like vitamin. The further you are from the equator, the lower your levels are likely to be. Low levels are also associated with many other diseases, including cancer, and so it is a good idea to get your levels checked, particularly if you find that you are one of those people who seem to get every cold or other bug that comes around.