Vitamin D to prevent colds

My family doctor advised me long time ago to take some vitamin D supplements. (Back then I had exams and did not want to get sick). He said that one of the reasons why people get sick more often during the winter months is the lack of vitamin D, which is really important for your immune system. During the summer months you get lots of vitamin D from the sun rays which go to your skin, but in the winter the days are shorter, the UV light is weaker and you wear more clothes and the sun is not out as much either. I now take supplements with calcium and vitamin D3 and haven’t had a cold in more than 5 years, in addition you get stronger bones.