Bill Gottlieb, America’s #1 natural health coach and author of Bottom Line’s Speed Healing, suggests that vitamin D is a potent cold-and-flu fighter. Taking up to 5,000 IU (International Units) a day can make you far less likely to get a viral illness. In the winter, when people are less likely to get out in the vitamin D–producing sunlight, incidence of colds and flus spike, which further emphasizes the need for vitamin D supplements in the winter. At the first hint of a cold, take even more vitamin D (as determined by your body weight) for three days to prevent the virus from taking hold. Vitamin D-3 is the natural, preferred form of the vitamin.