And in the dark and cold winter months, when so little of the sun and all sorts of colds and viruses diseases await around every corner, and in the hot and sunny summer, when it is so easy to burn in the scorching sun, the most important factor for the maintenance of immunity is a high level of vitamin D in the body, which depends on the amount of ultraviolet radiation received. On this subject numerous studies have been conducted, which often contradict with the data, once considered immutable fact and the resulting recommendations are often diametrically opposed to those given earlier. You've probably seen some vague guidelines, such as the recommendation to be in the sun, "a few minutes every day." But these recommendations are too general to be useful. The amount of the sunlight, which is necessary to satisfy the requirement for vitamin D varies greatly depending on your location, skin type, season, time of day, and even atmospheric conditions.