The link between susceptibility to seasonal diseases in winter and low levels of vitamin D has also been proven in a study about the flu.
The group of schoolchildren taking vitamin D3 (30 micrograms per day from December to March) caught the flu by 8% less than the placebo group. Vitamin D3 provided special protection from the flu for children who had previously never supplemented this vitamin. Another significant observation was that for asthmatic children the number of asthma attacks caused by the flu was significantly reduced while supplementing this vitamin1.
1Randomized trial of vitamin D supplementation to prevent seasonal influenza A in schoolchildren.Am J Clin Nutr. 2010 May