## TB Transmission Routes **Key Point:** Pulmonary TB is transmitted primarily through **airborne droplet nuclei** (1–5 micrometers) expelled during coughing, sneezing, or speaking by patients with active respiratory TB. This is the dominant mode of transmission globally and in India. **High-Yield:** Droplet nuclei can remain suspended in air for 2–8 hours and travel several meters, making TB a highly contagious airborne infection. A single cough can generate ~3,000 droplet nuclei; a sneeze can generate ~40,000. **Mnemonic:** **AIR-TB** — **A**irborne, **I**nhalation, **R**espiratory route is the primary transmission mechanism. ## Comparative Routes of TB Transmission | Route | Organism | Frequency | Clinical Form | | --- | --- | --- | --- | | **Airborne (droplet nuclei)** | *M. tuberculosis* | ~90% of TB cases | Pulmonary TB | | Ingestion (milk/food) | *M. bovis* | ~2–3% (India) | Extrapulmonary TB (lymph nodes, GI) | | Percutaneous | *M. tuberculosis* | Rare | Cutaneous TB, occupational | | Transplacental | *M. tuberculosis* | <1% | Congenital TB | **Clinical Pearl:** Patients with **smear-positive pulmonary TB** (AFB-positive sputum) are the most infectious and pose the greatest transmission risk. Smear-negative cases have lower infectivity but can still transmit disease. **Warning:** ~~Ingestion of contaminated milk is the primary route~~ — While *M. bovis* transmission via unpasteurized milk occurs, it accounts for only 2–3% of TB cases in India and is NOT the most common route. Pasteurization and improved dairy hygiene have reduced this further.
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