What causes TB?
TB is caused by a bacterium called Mycobacterium tuberculosis. TB that affects the lungs is the most contagious type, but it usually only spreads after prolonged exposure to someone with the illness. For example, it often spreads within a family who live in the same house. In most healthy people, the immune system (the body's natural defence against infection and illness) kills the bacteria, and you have no symptoms. Sometimes the immune system cannot kill the bacteria, but manages to prevent it spreading in the body. This means you will not have any symptoms, but the bacteria will remain in your body. This is known as "latent TB". If the immune system fails to kill or contain the infection, it can spread within the lungs or other parts of the body and symptoms will develop within a few weeks or months. This is known as "active TB". Latent TB could develop into an active TB infection at a later date, particularly if your immune system becomes weakened.