The major natural source of the vitamin is synthesis of cholecalciferol in the lower layers of skin epidermis through a chemical reaction that is dependent on sun exposure (specifically UVB radiation).

In general, vitamin D functions to activate the innate and dampen the adaptive immune systems.[81] Deficiency has been linked to increased risk or severity of viral infections, including HIV.[82][83]

Results: 73.07% of the Greek and 69.28% of the Cypriot subjects of the sample had inadequate levels of vitamin D. The mean 25(OH)D value for the Greek subjects was found 25.08 ng/ml and for the Cypriots 25.37 ng/ml. The highest levels in both databases were recorded for the month September. For, both databases the multivariate linear regression models demonstrated that age and month were significantly associated with 25(OH)D levels.