Tuberculosis (TB) is caused by a type of bacterium called Mycobacterium tuberculosis.
It's spread when a person with active TB disease in their lungs coughs or sneezes and someone else inhales the expelled droplets, which contain TB bacteria.
You would have to spend prolonged periods (several hours) 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 for you to become infected by sitting next to an infected person on, for instance, a bus or train.
Not everyone with TB is infectious. Children with TB or people with a TB infection 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.
But in some cases, the bacteria infect the body but do not cause any symptoms (latent TB), or the infection begins to cause symptoms within weeks, months or even years (active TB).
Up to 10% of people with latent TB eventually develop active TB years after the initial infection.
This usually happens either within the first year or two of infection, or when the immune system is weakened – for example, if someone is having chemotherapy treatment for cancer.
Who's most at risk?
Anyone can get TB, but those at greatest risk include people:
- who live in, come from, or have spent time in a country or area with high levels of TB] – around 3 in every 4 TB cases in the UK affect people born outside the UK
- in prolonged close contact with someone who's infected
- living in crowded conditions
- with a condition that weakens their immune system, such as diabetes
- having treatments that weaken the immune system, such as chemotherapy or biological agents
- 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