This document summarizes epidemiological evidence that vitamin D, produced by exposure to UVB radiation, helps prevent cancer and other diseases. Ecological studies have found inverse correlations between UVB exposure and cancer rates. Cohort studies have linked higher vitamin D levels to reduced cancer risk. A randomized controlled trial found vitamin D and calcium supplementation reduced total cancer incidence. Studies also suggest vitamin D may help prevent respiratory infections, autoimmune diseases, and metabolic diseases. Maintaining vitamin D levels through moderate sun exposure or supplementation could significantly reduce disease rates and economic burdens.