Selenium deficiency occurs when the body does not get enough selenium, an essential trace mineral important for various bodily functions. Selenium plays a crucial role in metabolism, immune function, and the production of antioxidant enzymes. A lack of selenium can lead to health issues, including weakened immune response and increased risk of certain diseases. Symptoms of selenium deficiency may include fatigue, muscle weakness, and cognitive decline. In severe cases, it can lead to conditions like Keshan disease, a type of heart disease, and Kashin-Beck disease, which affects bones and joints. Ensuring adequate selenium intake through diet or supplements can help prevent these issues.