Infants and children who have regular contact (such as living in the same house) with someone who has active (infectious) tuberculosis (TB) have an increased risk of becoming infected. It is extremely rare for a fetus to become infected before birth.

Infants' immune systems quickly produce the tiny capsules (tubercles) that surround and wall off (encapsulate) TB-causing bacteria in the lungs. This process may cause extensive lung damage. Often a TB infection in infants quickly becomes active TB. In infants, it is also common for the disease to affect other parts of the body in addition to the lungs. Therefore, an infant who is found to have a TB infection needs to be treated as soon as possible. TB usually does not spread outside the lungs in older children unless they have weakened immune systems.

Children who have active TB who complete an entire course of treatment usually recover fully.