How does contagion differ between smear-positive and smear-negative TB?

• A. Smear-negative TB is more contagious
• B. Smear-positive TB is more contagious
• C. Both equally contagious
• D. Neither contagious

Answer: (B)

Contagion in TB largely follows how many bacilli are present in the sputum and how easily they are spread when the person breathes, coughs, or sneezes. Smear-positive TB means there are many acid-fast bacilli visible on smear microscopy, so coughing releases a large number of bacteria into the air. This makes transmission to others more likely in shared air spaces or with close, prolonged contact. Smear-negative TB has far fewer bacilli in the sputum, so fewer bacteria are expelled with coughing, reducing the chance of spreading infection. However, transmission can still occur with smear-negative disease under favorable conditions (long exposure, close contact, or cavitary disease).