Tuberculosis - Causes

Authored by Dr Laurence Knott, 05 Jul 2017

Reviewed by:
Dr Adrian Bonsall, 05 Jul 2017

Tuberculosis (TB) is common in developing countries. The death rate from TB is falling but it still causes about two million deaths per year worldwide. The main factors contributing to TB in the developing world are:

  • Poor nutrition.
  • Poor housing.
  • Poor general health.
  • Insufficient healthcare.
  • AIDS (TB is common in people with AIDS).

The number of people with active TB was at an all-time low in the UK in the early 1980s but the number of cases then increased. This was probably due to a combination of factors. These include:

  • An increase in poverty.
  • The ageing population.
  • Immigration from areas where TB is common.
  • More people travelling to areas in the world where TB is common.
  • AIDS.

In the last four years, however, the rate has started to fall. It is thought this is due to several factors, including tackling poor housing and poverty and identifying and treating TB brought into the UK by immigrants. In England in 2015, just under 6,000 new cases were reported. Rates of TB vary in different parts of the UK, with some London boroughs having very high rates.

Anyone can get TB. The risk is increased where one or more of the following apply:

  • Close contacts of a person who has active TB in the lungs (living in the same household, or spending a lot of time with that person).
  • If you or your family come from a country where TB is common.
  • Environment and poverty: rates of TB are higher among homeless people, prisoners, in large cities and in more deprived areas.
  • A poor immune system: for example, due to HIV infection, immune-suppressing treatment, or alcohol or drug dependency.
  • Malnutrition: poor nutrition and lack of vitamin D are linked to TB.
  • Age: babies, young children and the elderly are more susceptible to TB.

Most cases affect the lungs at first. TB bacteria are coughed or sneezed into the air by people with active TB disease. The bacteria are carried in the air in tiny water droplets. If you breathe in some TB bacteria, they may multiply in your lung. There are then three ways the infection may progress.

Lungs and airways with TB scar

1. Minor infection with no symptoms - occurs in most cases

Most people in good health who breathe in TB bacteria do not develop active TB disease. The TB bacteria that you breathe in begin to multiply in the lung. This stimulates your body defences (immune system) into action. The TB bacteria are killed or made inactive by the immune system. There may be some mild symptoms for a short time, or no symptoms, and the infection is halted.

You are not usually aware that you have had this mild infection. A small scar on the lung may be seen on a chest X-ray. This may be the only indication of a previous TB infection.

2. Infection progressing into active TB disease - occurs in some cases

Active TB disease with symptoms occurs in some people who breathe in some TB bacteria. In these people the immune system does not win the battle and halt the invading bacteria. The TB bacteria multiply further and spread to other parts of the lung and body. Symptoms of active TB then develop about 6-8 weeks after first breathing in some bacteria.

TB infection which progresses to active disease can occur in anybody who is infected with TB bacteria. However, it is more likely if you are already in poor health. For example, it is common in malnourished children in developing countries. Newborn babies are also more at risk of active TB.

3. Secondary (reactivated) infection causing active disease

Some people develop active TB months or years after a minor TB infection had been halted. The body's immune system at first stops the bacteria from multiplying (as above). However, not all the bacteria may be killed. Some bacteria may be 'walled off' in the scar tissue of the initial minor infection. They are stopped from multiplying by the immune system. They do no harm but can remain inactive for many years. The inactive TB bacteria may later start to multiply and cause active TB if the body's immune system becomes weaker for some reason. A weak immune system and re-activated TB are more likely to occur if you:

  • Are elderly or frail.
  • Are malnourished.
  • Have diabetes.
  • Take steroids or immunosuppressant medication.
  • Have kidney failure.
  • Are alcohol-dependent.
  • Have AIDS.

As mentioned above, the disease that we call 'tuberculosis', or 'TB', is caused by a bacterium called Mycobacterium tuberculosis. Other bacteria in the same family are called Mycobacterium bovis and Mycobacterium africanum. They are rare causes of infection in the UK. Mycobacterium bovis used to be more common, as it is passed on from contaminated unpasteurised milk or from infected cattle (now rare in the UK). Treatment for these infections is mostly similar to that for standard TB.

There are various other bacteria in the mycobacterium family which are called atypical mycobacteria. Most of these do not cause infection. However, they sometimes cause serious infection in people whose immune system is not working very well. (For example, some people who have AIDS.) Treatment is with long courses of antibiotics.

Further reading and references

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