M. tuberculosis or M. bovis infection most commonly causes disease in the lungs, but any part of the body can be affected, particularly the lymph nodes.
The initial infection with M. tuberculosis usually goes unnoticed, and most of those infected enter a latent phase (LTBI). The lifetime risk for infected people progressing from this latent phase to active TB disease is as high as 20 percent, but this risk is strongly affected by the age of the person, the presence of healed lesions on chest X-ray and immunosuppression.2, 3
When TB disease does occur, clinical manifestations most often appear one to six months after infection. The most common site of infection is the lung (pulmonary TB), where TB infection classically causes an asymmetrical pulmonary infiltrate, which undergoes caseation, cavity formation and fibrosis if it progresses. Young children with active TB disease may be asymptomatic or present with symptoms of fever, lassitude and cough. Older children and adults with active TB disease may present with symptoms of anorexia, fatigue, weight loss, chills, night sweats, cough, haemoptysis and chest pain.
Any organ can be affected by extrapulmonary TB, causing meningitis, pleurisy, pericarditis, bone or joint infection, renal infection, gastrointestinal tract infection, peritonitis or lymphadenitis, or disseminating via the bloodstream and affecting multiple organs (disseminated TB). Disseminated and meningeal TB are more common in very young children.