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Transmission
People who have active TB – TB that the immune system has not been able to control – in the lungs can transmit the disease to other people. TB is spread through the air when a person with active TB coughs.
Rarely, TB can affect the larynx, and active TB here can be spread when a person coughs, shouts, or sneezes. Occasionally, TB can be spread from open wounds.
However, not everybody who has active TB is infectious.
You need to have close contact with TB in an enclosed space, such as an aeroplane or stuffy room, for at least eight hours to run a substantial risk of becoming infected with TB.
In the UK, there have been cases of HIV-positive people becoming infected with TB on hospital wards. This happened before it was realised that TB was a major health concern for people with HIV, and it is now standard practice for HIV-positive people with TB to be cared for in single rooms rather than wards, and for these rooms to have "negative pressure" meaning that the air is gently sucked out of the room and expelled outside the building so it cannot escape into the rest of the hospital.
In very rare cases, TB can cause ulcers to develop on the skin, and these can be infectious.