Causes of tuberculosis

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

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

However, although it is spread in a similar way to cold or 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

Your immune system will usually be able to defeat 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 will begin to cause symptoms within weeks or months (active TB).

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

Increased risk

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

• those living in environments where the level of existing TB infection is higher than normal
• people with health conditions such as HIV or whose circumstances mean they are less able to fight off a TB infection

Other things that can increase your risk of developing an active TB infection include:

• being in close contact with someone who is infected
• having lived in, travelled to or had visitors from parts of the world where TB is common
• being part of an ethnic group that originated in parts of the world where TB is still common
• having a weakened immune system because of HIV, diabetes or other medical conditions
• having a weakened immune system because of long courses of medication, such as corticosteroids, chemotherapy or tumour necrosis factor blockers (used to treat some types of arthritis) 
• being very young or very old – the immune systems of people who are young or elderly tend to be weaker than those of healthy adults
• being in poor health or having a poor diet due to lifestyle and other problems, such as drug misuse, alcohol misuse or homelessness
• living in poor or crowded housing conditions, such as prisons