Transmission is by air droplet.
Sneezing, coughing or talking releases about 40,000 droplet (0.5-5 micrometer in diameter),
each capable of infection. When inhaled by unsuspecting person, process of
infection starts. The bacillus enters the lung to cause a non-specific
inflammatory response resulting in
Ghon’s focus and regional node inflammation (primary complex). Primary complex
heals spontaneously with fibrosis and calcification forming a granuloma where
the bacillus persists in a dormant form that can be reactivated any time.
Individual immunity is important in the response to
infection. About 90% of those infected have asymptomatic latent infection while
10% may progress from latent infection to the disease. Adult pulmonary tuberculosis
may therefore occur by reactivation of latent bacilli or exogenous
re-infection.
Body develops certain level of cell mediated immunity that
protects the individual. This immunity is destroyed by HIV/AIDS leading to
reactivation, hence increase of tuberclosis cases. Destruction of lung
paranchyma and cavitation (Tubercle) is important feature of the adult tuberculosis.
This is also called cell death (Necrosis) (K. Parks 2009).