Tuberculosis (TB) is caused by a type of bacterium called Mycobacterium tuberculosis.

The condition is spread when a person with an active TB infection in their lungs coughs or sneezes and someone else inhales the expelled droplets, which contain TB bacteria.

Although it is spread in a similar way to a cold or the flu, TB is not as contagious. You would usually have to spend prolonged periods in close contact with an infected person to catch the infection yourself.

For example, TB infections usually spread between family members who live in the same house. It would be highly unlikely to become infected by sitting next to an infected person on a bus or train.

Not everyone with TB is infectious. Generally, children with TB or people with TB that occurs outside the lungs (extrapulmonary TB) do not spread the infection.

Latent or active TB

In most healthy people the immune system is able to destroy the bacteria that cause TB. However, in some cases the bacteria infect the body but don't cause any symptoms (latent TB), or the infection begins to cause symptoms within weeks or months (active TB).

Up to 10% of people with latent TB eventually develop active TB years after the initial infection. This usually happens when the immune system is weakened – for example, during chemotherapy.

Who's most at risk?

Anyone can catch TB, but people particularly at risk include those:

• who live in, come from, or have spent time in a country or area with high levels of TB – around three in every four TB cases in the UK affect people born outside the UK
• in prolonged close contact with someone who is infected
• living in crowded conditions
• with a condition that weakens their immune system, such as HIV
• having treatments that weaken the immune system, such as corticosteroids, chemotherapy or tumour necrosis factor (TNF) inhibitors (used to treat some types of arthritis and certain gut conditions)
• who are very young or very old – the immune systems of people who are young or elderly tend to be weaker than those of healthy adults
• in poor health or with a poor diet because of lifestyle and other problems, such as drug misuse, alcohol misuse, or homelessness