A Ghon focus is a primary lesion caused by mycobacterium

Mycobacterium tuberculosis

Mycobacterium tuberculosis is a pathogenic bacterial species in the genus Mycobacterium and the causative agent of most cases of tuberculosis . First discovered in 1882 by Robert Koch, M...

 bacilli

Bacilli

Bacilli refers to a taxonomic class of bacteria. It includes two orders, Bacillales and Lactobacillales, which contain several well-known pathogens like Bacillus anthracis .-Ambiguity:...

 (tuberculosis

Tuberculosis

Tuberculosis, MTB, or TB is a common, and in many cases lethal, infectious disease caused by various strains of mycobacteria, usually Mycobacterium tuberculosis. Tuberculosis usually attacks the lungs but can also affect other parts of the body...

) developed in the lung of a previously uninfected individual. It is named for Anton Ghon

Anton Ghon

Anton Ghon was an Austrian pathologist who was a native of Villach. In 1890 he earned his medical degree in Graz, and afterwards spent several years at the pathological institute in Vienna, where he worked with Anton Weichselbaum...

 (1866-1936), an Austrian pathologist.

It is a small area of granulomatous inflammation, only detectable by chest X-ray

Chest X-ray

In medicine, a chest radiograph, commonly called a chest X-ray , is a projection radiograph of the chest used to diagnose conditions affecting the chest, its contents, and nearby structures...

 if it calcifies or grows substantially (see tuberculosis radiology

Tuberculosis radiology

Radiology is used in the diagnosis of tuberculosis.-Chest X-ray:A posterior-anterior chest X-ray is the standard view used; other views or CT scans may be necessary....

). Typically these will heal, but in some cases, especially in immunosuppressed

Immunosuppression

Immunosuppression involves an act that reduces the activation or efficacy of the immune system. Some portions of the immune system itself have immuno-suppressive effects on other parts of the immune system, and immunosuppression may occur as an adverse reaction to treatment of other...

 patients, it will progress to miliary tuberculosis

Miliary tuberculosis

Miliary tuberculosis is a form of tuberculosis that is characterized by a wide dissemination into the human body and by the tiny size of the lesions...

 (so named due to the granulomas resembling millet

Millet

The millets are a group of small-seeded species of cereal crops or grains, widely grown around the world for food and fodder. They do not form a taxonomic group, but rather a functional or agronomic one. Their essential similarities are that they are small-seeded grasses grown in difficult...

 seeds on a chest X-ray).

The classical location for primary infection is surrounding the lobar fissures, either in the upper part of the lower lobe or lower part of the upper lobe.

If the Ghon focus also involves infection of adjacent lymphatics and hilar lymph nodes, it is known as the Ghon's complex

Ghon's complex

Ghon's complex is a lesion seen in the lung that is caused by tuberculosis. The lesions consist of a calcified focus of infection and an associated lymph node...

 or primary complex. When a Ghon's complex undergoes fibrosis and calcification it is called a Ranke complex..