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Mycobacterium tuberculosis, a slow-growing acid-fast aerobic rod, causes tuberculosis. It spreads through respiratory droplets released when infected individuals breathe, talk, sing, sneeze, or cough. As droplets evaporate, tiny nuclei measuring 1-5 µm remain suspended in air for minutes to hours, transmitting infection when inhaled by others.

TB transmission requires close, frequent, or prolonged exposure. Brief contact with few tubercle bacilli rarely causes infection. Transmission probability depends on microorganism concentration, exposure duration, ventilation conditions, and the individual's immune system. TB cannot spread through touch, food sharing, kissing, or physical contact.

While TB primarily affects lung parenchyma, it can also involve the meninges, kidneys, bones, and lymph nodes. The disease spreads systemically through airborne droplet nuclei inhaled into the respiratory tract, potentially seeding infection in multiple organ systems throughout the body.

Healthcare workers performing high-risk procedures face increased TB exposure. These include administering aerosolized medications, bronchoscopy, suctioning, caring for immunocompromised patients, and anesthesia-related procedures like intubation. Risk depends on exposure duration, proximity to infected patients, and ventilation conditions in clinical environments.

Immunocompromised individuals, including HIV-positive persons, organ transplant recipients, those undergoing hemodialysis, and people with long-term diabetes, have weakened immune systems unable to effectively fight TB infection. Substance use disorders and prolonged corticosteroid therapy similarly impair immune function, significantly increasing TB development risk.

Overcrowded and substandard housing, institutionalization, and poorly ventilated spaces like prisons increase TB concentration and transmission. Immigration from or recent travel to high-prevalence countries, limited healthcare access, malnourishment, chronic kidney disease, and diabetes also elevate TB risk through environmental exposure and compromised health status.

Untreated HIV causes substantial reduction in CD4+ T-cells critical for immune response, creating severe immunosuppression. This makes individuals far more vulnerable to opportunistic infections including TB. Intravenous drug use increases HIV transmission risk through needle sharing, compounding TB susceptibility in affected populations.