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What is tuberculosis?

Tuberculosis (TB) is a disease caused by a bacterium called Mycobacterium tuberculosis. On this page, we explain the difference between active and latent TB, who is most at risk and what you should do if you think you’ve been exposed to TB.

Anyone can catch TB by breathing in TB bacteria. These bacteria are in tiny droplets in the air coughed out by people with TB in their lung. In most people, if you breathe in TB bacteria your immune system – your body’s natural defence - will control most of the bacteria and you will not get ill.

However, if you do become ill, which can happen weeks, months or even years after you breathe in TB bacteria, this is called active TB.

In most people, the body's immune system controls the TB bacteria, which stay in the body at a low level. You won’t get ill and you’re not infectious. This is called latent TB.

In about five to ten out of every 100 people with latent TB, the TB bacteria can start to multiply again or reactivate and lead to symptoms of active TB.

Who is most at risk of developing TB?

You’re most at risk of developing active TB if your immune system isn’t working well. For example, you may have a condition like diabetes or HIV. If you get a confirmed diagnosis of TB, you will usually have tests for these conditions too. Or you may have had an organ transplant or treatment for conditions like cancer and rheumatoid arthritis.

Excessive alcohol and drug use and smoking are also risk factors for developing active TB.

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Last medically reviewed: September 2020. Due for review: September 2023

This information uses the best available medical evidence and was produced with the support of people living with lung conditions. Find out how we produce our information. If you’d like to see our references get in touch.