When you feel a cold coming on start popping zinc lozenges. That will do much more for you than vitamin C or Echinacea.
Although some observational studies have suggested that vitamin D can help prevent upper respiratory ailments in adults, no such data substantiates that claim for children. A new study that compared low- and high-dose vitamin D with respect to such problems in kids found no effect at either level. Sorry, parents!
Compared to warm winters, cold winters are likelier to land more people in the hospital, particularly the emergency room.