Mycobacterium tuberculosis (M. tb) is a species of pathogenic bacteria that cannot be Gram stained. Primarily a pathogen of the mammalian respiratory system, it infects the lungs. It is the causative agent of tuberculosis.
Tuberculosis generally affects the lungs, but can also affect other parts of the body. Most infections do not have symptoms, in which case it is known as latent tuberculosis.
Classic symptoms of active TB are a chronic cough with blood-containing mucus, fever, night sweats, and weight loss. Infection of other organs can cause a wide range of symptoms. People with latent TB do not spread the disease. Active infection occurs more often in people with HIV/AIDS and in those who smoke.