The BCG vaccine is a vaccine primarily used against tuberculosis (TB). It is made from a weakened strain of the bacterium Mycobacterium bovis, which is related to the bacteria that cause TB in humans. The vaccine is most effective in preventing severe forms of TB, especially in children, and is often administered in countries where TB is common. The BCG vaccine is typically given shortly after birth or during early childhood. While it does not guarantee complete protection against TB, it significantly reduces the risk of developing severe disease. In some regions, it is part of routine immunization programs to help control the spread of TB.