Vitamin D from sunshine if possible. Or D3 supplements. It is believed that we get more colds and flu in the winter due to lower sun exposures, which reduces our skin production of vitamin D. Vitamin D is an immune modulator -- makes your immune system works better. Since there is no cure for cold, the best one can do is to nurture and support the body's own healing system. That means provide as much raw ingredients and nutrients to the body so that it can use them to do its thing.