drawn into the alveoli of their lungs, which initiates an immune system response. If the immune
system kills the bacteria, they are cleared from the body but, if not, the body becomes infected with TB.
The body’s immune response is often able to contain and inactivate TB bacteria; the person is then said to have latent TB infection (LTBI). This is asymptomatic and not infectious, so it cannot be passed on; however, there is a 10% lifetime chance that the bacteria can reactivate and cause TB disease. Activated LTBI is most likely to occur in the first two years of infection, or in patients who are immunocompromised by other illness or old age (Williams, 2008). If the immune system is unable to kill or contain the bacteria, they can continue to grow and multiply, causing sickness. This is called active TB disease.