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TB - the basics
Many people are exposed to TB as children, when they breathe in TB germs that have been expelled into the air from the lungs of a person infected with TB.
TB can have a number of phases.
The TB germs multiply in the lungs, cause inflammation, and move to the lymph glands – the command centres of the immune system - in the lungs. This is phase of this illness is called primary TB. The TB can spread and grow and cause disease. In most HIV-negative people, the body's immune system contains the TB by forming a wall of scar tissue around it. Although a person may not feel ill because of TB, the TB germ can remain alive within this contained area for many years, even decades, causing illness at a later time. This type of TB is usually called latent TB.
In HIV-positive people, TB may not get walled off by scar tissue and spread, causing disease.
TB that is causing illness is called active TB.
TB that resurfaces from scar tissue and causes illness after a period of time is called reactivation TB.
In people who are HIV-negative, the lifetime risk of latent TB becoming active is about one in ten. However, in untreated people with HIV, the risk increases to between five and ten per cent per year, giving a lifetime risk of 50% or more.
Unlike most other opportunistic infections (so called because they take the opportunity of the body’s weakened immune defences to develop) seen in people with HIV, TB can occur in people with relatively high CD4 cell counts and can be transmitted to other people, HIV-positive and HIV-negative.