introduction: vitamin d deficiency (vdd) is a common condition among several populations in the world. vdd is associated with higher incidence of immune system disorders and faster progression of some infectious diseases. vitamin d is known to be of physiological importance, it is considered an essential micronutrient for the bone health and plays a beneficial role in the prevention and/or treatment of a number of chronic diseases. vitamin d has a complex action on the immune system. results: evidence that vitamin d protects against tuberculosis has been supported by in vitro, epidemiological and some preliminary clinical studies. vitamin d has a potential effect on hiv (human immunodeficiency virus) and plays a crucial role in the defence against respiratory infections. conclusions: vitamin d supplementation could be a low-cost, practical method to protect groups of people with high incidence of those diseases. public health education should stress the need for adequate dietary intake of vitamin d in those vulnerable groups.