Vitamin D, in the form of sunlight, is converted into calcium for stronger bones and a healthier immune system. The synergistic effect of vitamin D along w/other vitamins/minerals obtained from food sources (ideally) builds a strong immune defense so one is not as susceptible to viruses/bacteria which are opportunists and seek a weak terrain (the human body). My grandfather was a well-known and highly respected doctor in the late 1800's - 1900's who was very familiar w/the health-giving benefits of sunlight. He advocated exposure to the sun (no sunscreen) and water as the cheapest medicine there is. Solariums were in favor, mostly in Europe, and people who were ill would seek the mineral-rich waters of springs and the benefits of sunshine. My own father swore by the sun and every day would lie in the sun for as long as possible, even in the winter. He lived into his ripe old 80's and died a peaceful death from natural causes.