Tuberculous infection usually starts when TB bacilli are inhaled into the lung. During the first 6 weeks of infection the immune system is unable to control the TB bacilli, which multiply and cause a small, local area of inflammation in the lung. From here TB bacilli also spread along the lymphatics to the local lymph nodes (hilar nodes) at the place where the main bronchi divide into branches. After 6 weeks the immune system usually becomes active and kills most of the TB bacilli. As a result, the primary infection is asymptomatic in most children and does not cause clinical illness. Primary tuberculosis is common.
The primary infection in the lung, together with the infected regional lymph nodes, is called the primary complex (Ghon complex). Often primary infection of the lung can only be recognised by the presence of a positive skin test. Mild flu-like symptoms may sometimes occur.
Inhaling tuberculous bacilli into the lung results in a primary infection.