TB is caused by the bacillus Mycobacterium tuberculosis and is spread through the air when someone who has infectious TB coughs or sneezes. It may then enter the lungs of someone nearby.
The average person with untreated, infectious TB will infect 10 others per year.
Not everybody who inhales TB germs becomes ill. Someone with normal immunity may reject the infection and stay healthy. When the immune system cannot overcome the infection, the person then becomes ill.