TB is transmitted by respiratory droplets through sneezing or coughing by an infected person. Most infected persons have had a sustained exposure to the active agent, rather than a single one.

The M. tuberculosis bacilli are inspired into the respiratory tract and usually lodge in the lower part of the upper lobe or the upper part of the lower lobe. The TB bacilli need high levels of oxygen to survive. When they reach the lungs, they multiply rapidly.

Mycobacteria that are not destroyed lie dormant until there is a decrease in the host's resistance. Of individuals who inhale mycobacteria 5% develop clinical TB at that time, 95% have been infected and have no clinical symptoms but enter a latent phase and are at risk to develop TB later.

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