A Ghon focus (also called a primary or Ranke complex) is a lesion caused by mycobacterium bacilli developed in the lung of a previously uninfected individual. It is named for Anton Ghon (1866-1936), a Czech pathologist.

It is a small area of granulomatous inflammation, only detectable by chest X-ray if it calcifies or grows substantially (see tuberculosis radiology). Typically these will heal, but in some cases, especially in immunosuppressed patients, it will progress to miliary tuberculosis (so named due to the calcified granulomas resembling millet seeds on a chest X-ray).

The classical location for primary infection is in the upper lobe of the lung, with associated infection of surrounding lymph nodes.

Tuberculosis | Bacterial diseases