Tuberculosis is caused by a bacteria known as Mycobacterium tuberculosis. This bacteria contaminates and spreads through the air. So when an infected person coughs, sneezes, speaks or sings, those without are likely to get infected also.
Children are at high risk of developing active tuberculosis because of a poorly developed immune system. When a child develops active tuberculosis disease, there is a family member or close contact with TB. As they say: "TB in a child means TB in an adult."