TB

Mycobacterium tuberculosis is inhaled into the alveoli, where alveolar macrophages engulf it. Unlike many bacteria, M. tuberculosis can survive inside these cells, allowing the infection to persist while the immune system organizes a granuloma around it. A granuloma is containment, not necessarily eradication: viable organisms may remain dormant for years. When cellular immunity keeps the infection contained, the person has latent TB infection rather than active disease. Latent infection usually causes no symptoms and does not transmit TB. If immune control weakens—for example, with HIV, immunosuppressive therapy, malnutrition, diabetes, kidney disease, or advanced age—the organisms may multiply and produce active TB. Pulmonary or laryngeal TB spreads through airborne droplet nuclei released during coughing, speaking, or singing. This is why a patient with suspected infectious pulmonary TB requires airborne precautions, not merely routine contact precautions. TB can also affect lymph nodes, bone, the kidneys, or the central nervous system; extrapulmonary disease may be serious even when respiratory transmission is not the main concern.