The person will have a positive TB skin test but not be affected by the bacteria. If the immune system begins to work less well because of ageing, poor nutrition or living conditions, or infections such as HIV or other causes, the bacteria can break out of the tubercles and spread to other areas of the lungs . The immune system fights back, killing bacteria and the surrounding cells. The earliest symptoms start at about this stage. Over time more and more of the lungs are damaged.
Cavities can form and break through into the bronchi, releasing bacteria that can be coughed out.
The single most important preventive measure is good ventilation. Good air circulation greatly reduces the likelihood of inhaling enough bacteria to become infected. The second most important measure is identifying and treating active TB cases and testing and treating all persons who might have been infected. According to the World Health Organization, TB is the number one cause of death in the world. An estimated 1.7 billion people one-third of the world's populationare infected with TB bacteria.