Tuberculosis (TB) is an communicable disease usually caused by tubercle bacillus (MTB) bacteria. Tuberculosis generally affects the lungs, but also can affect other parts of the body. Most infections show no symptoms, in which case it is known as latent tuberculosis. Tuberculosis is spread from one person to subsequent through the air when people that have active TB in their lungs cough, spit, speak, or sneeze. People with latent TB don't spread the disease. Diagnosis of latent TB relies on the tuberculin diagnostic test (TST) or blood tests. Prevention of TB involves screening those at high risk, early detection and treatment of cases, and vaccination with the bacillus Calmette-Guérin (BCG) vaccine. General signs and symptoms include fever, chills, night sweats, loss of appetite, weight loss, and fatigue. Significant nail clubbing may also occur. Diagnosing active tuberculosis based only on signs and symptoms is difficult, as is diagnosing the disease in those that have a weakened system. A diagnosis of TB should, however, be considered in those with signs of lung disease or constitutional symptoms lasting longer than fortnight .