How TB progresses

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After inhaling the TB bacteria from the air, they can settle in your lungs. The bacteria enter via the respiratory mucous membranes and multiply to form a primary lesion.

While the main infection site is the lung, any organ can become infected if the bacteria spread.
They can enter the bloodstream and lymphatic system before travelling to other parts of the body.

Once you are infected by the TB bacteria, there are essentially two things that can happen: you may develop active disease; or your body may control the bacteria – you will be infected, but will not have active TB.

Most TB cases are pulmonary (in the lungs). However, TB can also break down bones and vertebrae, causing sufferers to become hump-backed. A rare form called "lupus vulgaris" attacks and disfigures the soft tissue of the face. TB can also attack the brain as a deadly form of meningitis. People infected with HIV have a higher incidence of TB in parts of the body other than the lungs.

TB Infection (Latency)

It is very important to realise that most people who are infected with the TB bacteria do not develop active TB and get TB disease.

These people therefore, have no symptoms and are not infectious. The immune system controls the infection by forming walls around the bacteria. This inactivates the bacteria, but does not kill them as they can lie dormant inside these walls for years.

In many people, TB bacteria remain inactive for a lifetime, but in others – especially those with weak immune systems – the bacteria may become active and cause active TB disease.

People with latent TB infection:

  • Have no symptoms
  • Do not feel sick
  • Cannot spread TB to others
  • Usually have a positive skin test reaction (see later)
  • Can develop TB disease later if they receive no preventive therapy or if their immune system decreases.

TB Disease

TB disease is a serious illness caused by active TB bacteria. It can either develop when you are first exposed to the TB bacteria, especially if you have a weak immune system, or it can develop as a reactivation disease in people who have been previously infected.

Some people develop TB disease within weeks of becoming infected because their immune systems are too weak to stop bacterial growth.

Other people may get sick later when their immune systems become weak for some reason such as drug abuse, poor nutrition, immune suppression, old age, ill health and HIV/AIDS.

TB bacteria become active if the immune system can't stop their growth; they multiply and cause disease.

Babies and children often are more at risk because of their developing immune systems. People with TB disease can be cured if they have adequate medical treatment. Without the correct treatment, however, they may become seriously ill and even die. 

Read more: 

What is tuberculosis? 

How can tuberculosis prevented? 

What is multidrug-resistant TB (MDR-TB)? 

Reviewed by Dr AW Dreyer, Pathologist and Clinical Microbiologist, Centre for Tuberculosis, National Institute for Communicable Diseases February 2015.

Previously reviewed by Joanna Evans, PhD, Molecular Mycobacteriology Research Unit, Division of Medical Microbiology Institute of Infectious Diseases and Molecular Medicine, University of Cape Town, February 2011.

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