Not everyone who is exposed to an infectious TB patient becomes infected with M. tuberculosis. Crowding in poorly ventilated rooms is one of the most important factors in transmission of tubercle bacilli, since it increases the intensity of contact with a case within the same household who has untreated pulmonary TB (PTB).

Although smear positive PTB is a strong predictor of infectiousness, studies have shown that a significant proportion of transmission occurs from people with smear-negative disease.