Tuberculosis is an infection caused by the bacterium Mycobacterium tuberculosis. It most commonly affects the lungs, but other parts of the body can be affected as well. Tuberculosis is classified into 2 types: latent tuberculosis and active tuberculosis. Active tuberculosis is highly contagious and is spread through the air. Symptoms of active tuberculosis include cough (sometimes with blood), fever, and weight loss. Latent tuberculosis differs from active tuberculosis in that it causes no symptoms and cannot be spread to others. This is because the body’s immune system has “walled off” the infection in a pocket inside the lung, keeping the bacteria dormant (“asleep”).