**Bacillus Calmette–Guérin (BCG)** is a live attenuated vaccine derived from *Mycobacterium bovis*, primarily used to prevent **tuberculosis (TB)**. It is widely administered in many countries as part of routine childhood immunization programs. In addition to its role as a vaccine, BCG is also used as an **immunotherapy for certain types of bladder cancer**, where it helps stimulate the body’s immune system to attack cancer cells. The drug works by enhancing the immune response against mycobacterial infections and malignant cells.