A child can only develop tuberculosis if the child is exposed to TB bacilli. Therefore, the more tuberculosis there is in the community, the higher is the risk that children will become infected. However, some children are at a particularly high risk of developing tuberculosis if they become infected with TB bacilli:

• Children exposed to large numbers of TB bacilli: Children living in poor, overcrowded, poorly ventilated conditions are at great risk of becoming infected. They are often exposed to large numbers of TB bacilli due to many adults with untreated pulmonary tuberculosis.
• Children with a weak immune system: In these children infection with TB bacilli may progress to tuberculosis because they have an inadequate immune system which is unable to control the infection.

Therefore, tuberculosis is most common when a child with a weak immune system is exposed to a large number of TB bacilli.