Summary: Interest in vitamin D, as well as its effects on a multitude of body processes, including the immune system, has grown significantly in past years. Many studies have shown a link between inflammatory diseases and low levels of vitamin D. Deficiency of this vitamin in the population is very common and this makes the topic so important. There are even talks of a worldwide epidemic of vitamin D deficiency and that is why widespread vitamin D supplementation is recommended. This article discusses the role of vitamin D in the proper functioning of the immune system and the results of studies on the consequences of its deficiency in selected infectious diseases.