I have found it to be true that keeping your vitamin D levels up will really help the most with cold and flu viruses. Vitamin D deficiency is actually in plague proportions in much of the world, even when people live in sub-tropics, because people avoid going in the sun. Now, I am one of those people, because I have always been one who burns easily, but taking natural vitamin D3 in high enough doses will actually help boost and maintain vitamin D levels.