Coloured X-ray (front view) of the chest in a 50 year old male patient, showing fibrosis in the lungs from old tuberculosis (more fibrous scarring, green, at upper left) and an infection of new tuberculosis (yellow, at right). Tuberculosis (TB) is a lung infection caused by the bacteria Mycobacterium tuberculosis, which can recur at periods when the patient's immune system is weakened.