Tuberculosis (TB) is an infectious disease caused by bacteria. It is caused by various types of mycobacteria, usually "Mycobacterium tuberculosis". The disease usually attacks the lungs, but it can also affect other parts of the body. The pathogen can travel through the air, and spread from one person to the next. Experts believe that one third of the world population is infected with "M. tuberculosis". New infections occur at a rate of one per second.