Tuberculosis (TB) is a potentially fatal contagious disease that can affect almost any part of the body but is mainly an infection of the lungs. It is caused by a bacteria Mycobacterium tuberculosis (MTB). Infection occurs when a person inhales droplet nuclei containing tubercle bacilli that reach the alveoli of the lungs. Most infections do not have symptoms; in which case it is known as latent tuberculosis. About 10% of latent infections progress to active disease when the person's immune system become weak due to malnutrition or diseases (like HIV). If it is untreated, TB kills about half of those infected. The classic symptoms of active TB are a chronic cough with blood-containing sputum, fever, night sweats, and weight loss.