What is tuberculosis?

Tuberculosis (TB) is a disease caused by a bacteria called Mycobacterium tuberculosis.

Anyone can catch TB by breathing in TB bacteria. These bacteria are in microscopic droplets in the air coughed out by people with TB in their lung. If you breathe in TB bacteria your immune system – your body’s natural defence - usually kills all the bacteria and you don’t get ill.  

If you 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.

You’re most at risk of developing active TB if your immune system isn’t working as well as normal. This may be because you have a condition like diabetes or HIV. These are usually tested for if TB is confirmed. Your immune system may also work less well after an organ transplant or treatment for conditions like cancer and rheumatoid arthritis.

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Last medically reviewed: September 2017. Due for review: September 2020

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.