2. How Tuberculosis is Transmitted?

Tuberculosis is spread from person to person by droplet nuclei that are produced when a person with pulmonary or laryngeal tuberculosis coughs/ sneezes and by aerosol-producing investigations such as bronchoscopy and sputum induction.

People with active tuberculosis generate droplets of different sizes. The larger droplets which contain higher numbers of bacilli do not remain airborne for long periods. If they are inhaled, they do not reach the alveoli because they are trapped by the mucous in the upper airway and from there transported by mucociliary action to the oro-pharynx and swallowed or expectorated. The smaller droplets which are 1 to 5 µm in diameter containing fewer (±1 - 5 bacilli), are highly infectious. They remain airborne for long periods of time in any indoor space. When inhaled they can easily reach the alveolar spaces within the lungs, where the organisms replicate. It is estimated that one cough can produce 3,000 droplet nuclei and a sneeze up to a million droplets; about 10 - 200 droplet nuclei are sufficient to cause infection. The most infectious people are those who have smear positive pulmonary TB (coughing up the bacilli), particularly with lung cavities. People with smear negative pulmonary TB cases are much less infectious and those with extra-pulmonary TB are almost never infectious, unless they have pulmonary tuberculosis as well.