TB also gets the nearby hilar lymph nodes, either carried over by immune cells through the lymph or by direct extension of the Ghon focus infection and causes caseation there as well.
If and when a person's immune system becomes compromised like with AIDS or with aging, the Ghon focus can become reactivated and the infection can spread to either one or both upper lobes of the lungs.
A Ghon focus is a primary lesion usually subpleural, often in the mid to lower zones, caused by mycobacterium bacilli (tuberculosis) developed in the lung of a nonimmune host (usually a child). It is named for Anton Ghon (1866–1936), an Austrian pathologist.