Tuberculosis (TB) is caused by an infection with a bacteria known as Mycobacterium tuberculosis. TB is a contagious disease that is airborne (meaning spreads through air). When a person with the disease coughs, sneezes, talks or spits, the TB bacilli are released into the air. On inhaling the bacilli, a healthy person becomes infected. Everyone, who gets infected, does not develop the disease or becomes sick. According to research, about 5% to 10% of people infected with TB become actively sick.
In some people with strong immune system, the immune system may fight the TB bacilli and kill it whereas in others, the immune system may wall off the germ where it can lie dormant (inactive) for years. These people have latent TB infection i.e. they carry the bacteria, but do not have any symptoms, but on Mantoux test, they tend to develop a positive reaction. If due to any reason their immunity is weakened, they may develop the disease due to reactivation of the dormant TB bacilli.
If the immune system fails to kill or contain the germ, infection with TB bacilli leads to active disease. In these people, the TB bacilli multiply and cause damage to the body. People, who have active TB, will become sick and are contagious (i.e. transmit the disease). The signs and symptoms may start a few weeks after infection with the TB bacteria or even years later.
In most cases, TB does not go away without treatment. Even if the symptoms of pulmonary tuberculosis go away without treatment, in 50%-60% of cases, the disease can return. Experts say that prognosis of people with untreated TB is far worse than those, who seek treatment. Estimates show that about 50 percent of people with untreated TB die within 5 years.