## TB Transmission Mechanism **Key Point:** Tuberculosis is primarily transmitted through airborne droplet nuclei expelled during coughing, sneezing, or speaking by persons with active pulmonary or laryngeal TB. **High-Yield:** Droplet nuclei are particles 1–5 micrometers in diameter that can remain suspended in air for hours and are the sole significant mode of TB transmission between humans. ### Airborne Transmission Process 1. Person with active TB disease releases droplet nuclei (1–5 μm) 2. Nuclei remain suspended in air for 2–8 hours 3. Susceptible person inhales infected nuclei 4. Nuclei reach alveoli → infection established 5. Risk increases with prolonged exposure in poorly ventilated spaces **Mnemonic:** **DROPLETS** = Diameter <5 μm, Respiratory origin, Oxygen-independent survival, Prolonged airborne time, Lodges in alveoli, Easily transmissible, Transmission via inhalation, Spread in closed spaces ### Why Other Routes Do NOT Occur | Route | Why Not TB | Organism Requirement | | --- | --- | --- | | Fecal-oral | MTB destroyed by gastric acid; no intestinal colonization | Acid-labile pathogens only | | Percutaneous | MTB does not survive in bloodstream without protective granuloma | Requires intracellular niche | | Transplacental | Rare; only if placental TB present (miliary TB) | Vertical transmission <1% | **Clinical Pearl:** Patients with smear-positive pulmonary TB are highly infectious; those with extrapulmonary TB (except laryngeal) are non-infectious because bacilli do not reach airways. [cite:Park 26e Ch 7; Harrison 21e Ch 205]
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