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Tuberculosis (TB) is transmitted from an infected person to a susceptible person in airborne particles, called droplet nuclei. These are 1–5 microns in diameter. These infectious droplet nuclei are tiny water droplets with the bacteria that are released when persons who have pulmonary or laryngeal tuberculosis cough, sneeze, laugh, shout etc. These tiny droplet nuclei remain suspended in the air for up to several hours. Tuberculosis bacteria, (Mycobacterium tuberculosis) however are transmitted through the air, not by surface contact. This means touching cannot spread the infection unless it is breathed in.
Transmission occurs when a person inhales droplet nuclei containing tuberculosis bacteria. These droplet nuclei travels via mouth or nasal passages and move into the upper respiratory tract. Thereafter they reach the bronchi and ultimately to the lungs and the alveoli.
The factors that determine the risk of transmission of tuberculosis bacteria include:-
Patients with high loads of the bacteria in their expelled sputum are more likely to transmit the infection than those with low numbers of bacteria.
An individual who is infectious will have certain features. This includes presence of cough lasting three weeks or more, those with involvement of the larynx, those who fail to cover their mouth and nose while coughing or sneezing and those on inappropriate and inadequate treatment of the infection.
Young children with pulmonary and laryngeal tuberculosis are less likely than adults to be infectious.
Persons who show presence of cavities in the lungs on X rays, have a positive culture for tuberculosis from their sputum and have a positive presence of the bacteria under the microscope after ZN stain are also infectious.