[A short excerpt that's interesting. Jan]
For decades, scientists have known that exposure to sunlight — specifically ultraviolet (UV) radiation — can suppress the immune system’s response to bacterial, viral and fungal infections. To suppress the human immune system, it takes UVR doses that are only 30 to 50 percent of what’s required to cause barely-detectable sunburn, according to a 2010 article in the Journal of Investigative Dermatology.
At the same time, however, sunlight causes the body to produce vitamin D. A recent study in Nature Immunology suggested that T-cells don’t mobilize if they detect only small amounts of vitamin D in the bloodstream. Additionally, other research suggests vitamin D might induce the production of anti-microbial peptides in the skin — these compounds help defend the body against new infections.