vitamin D is really good for colds/illness. make sure you take it with some fat food because its fat soluble. take more than what it calls for you cant really od on the stuff. vitamin c is also good for you but I wouldnt take it at the same time as the D.
My wife is a nurse, and works around sick people all day. This is what she does everyday to keep her immune system strong to ward off sickness. 2 super B vitamins, 2 echinacea with goldenseal root, 2 C vitamins, 2 vitamin D, and a good multi-vitamin. After you get better, wash your hands after touching anything someone else has touched, even your own family. Take one of those liquid germ killers with you everywhere you go.