• Close contacts are at the highest risk of becoming infected
• Infectiousness of the person with TB
• Environment in which the exposure took place
• Duration of exposure
• Virulence of the organism

Medical conditions that increase the risk of progression to TB disease

• HIV Infection
• Illicit drug and alcohol use
• Recent Infection
• Diabetes Mellitus
• Silicosis: disorder of lungs caused by prolonged exposure to inorganic compounds such as silicon dioxide found in sand, quartz and many other stones
• Prolonged use of Corticosteroids
• Immunosuppressive therapy
• Malnutrition

Patterns of Infection

There are two major patterns of disease with Tuberculosis:

• Primary Tuberculosis: seen as an initial infection, usually in children. The initial focus of infection is a small subpleural granuloma accompianied by granulomatous hilar lymph node infection. Together, these make up the Ghon complex. In nearly all cases, these granulomas resolve and there is no further spread of the infection.
• Secondary Tuberculosis: seen mostly in adults as a reactivation of previous infection (or reinfection), particularly when health status declines. The granulomatous inflammation is much more florid and widespread. Typically, the upper lung lobes are most affected, and cavitation can occur. TB can occur anywhere in the body, however it is more commonly associated with the lungs and pleura.

Dissemination of tuberculosis outside the lungs can lead to the appearance of a number of uncommon findings with characteristic patterns:

Skeletal Tuberculosis: Tuberculosis osteomyelitis involves mainly the thoracic and lumbar vertebrae(known as Pott's Disease) followed by knee and hip. There is extensive necrosis and bony destruction with compressed fractures and extension to soft tissues, including psoas "cold" abscess.

Genital Tract Tuberculosis: Tuberculosis salpingitis and endometritis result from dissemination of tuberculosis to the fallopian tube that leads to granulomatous salpingitis, which can drain into the endometrial cavity and cause a granulomatous endometritis with irregular menstrual bleeding and infertility. In the male , tuberculosis involves prostate and epididymis most often with non-tender induration and infertility.

Urinary Tract Tuberculosis: A "sterile pyuria" with white blood cells present in urine, but a negative routine bacterial culture may suggest the diagnosis of renal tuberculosis. Progressive destruction of renal parenchyma occurs if not treated. Drainage to the ureters can lead to inflammation with uretal stricture.

CNS Tuberculosis: A meningeal pattern of spread can occur, with the cerebrospinal fluid typically showing high protein, low glucose, and lymphocytosis. The base of the brain is often involved, and various cranial nerve signs may be present. Rarely, a solitary granuloma, or "tuberculoma", may form and manifest with seizures.

Gastrointestinal Tuberculosis: This is uncommon today because routine pasteurisation of milk has elimated Mycobacterium bovis infections. However, M. tuberculosis organisms coughed up in sputum may be swallowed into the GI tract. The classic lesions are circumferential ulcerations with stricture of the small intestine. There is a preference for ileocecal involvement because of the abuntant lymphoid tissue and slower rate of passage of lumenal contents.

Adrenal Tuberculosis: Spread of tuberculosis to adrenal glands is usually bilateral, so that both adrenal glands become enlarged. Destruction of cortex leads to Addison's disease.

Scrofula: Tuberculosis lymphadenitis of the cervical nodes may produce a mass of firm, matted nodes just under the mandible (jaw) similar to an abscess. There can be chronic draining fistulous tracts to overlying skin. This complication may appear in children, Mycobacterium scrofolaceum may be cultured.

Cardiac Tuberculosis: The pericardium is the usual site for tuberculosis infection of the heart. The result is a granulomatous pericarditis that can be haemorrhagic. If extensive and chronic, there can be fibrosis with calcification, leading to a constrictive pericarditis.