Vitamin D deficiency is a common health issue that occurs when a person does not have enough vitamin D in their body. This essential nutrient is important for maintaining strong bones, supporting immune function, and regulating cell growth. A deficiency in vitamin D can lead to a variety of health problems, such as weakened bones, increased risk of certain diseases, and mood disorders. Factors that contribute to vitamin D deficiency include inadequate sun exposure, poor diet, and certain medical conditions. Treatment typically involves increasing vitamin D intake through supplementation or dietary changes.