Exactly what causes Chronic Fatigue Syndrome (CFS) is unknown, but there are several theories.
Some experts think a viral infection such as glandular fever can trigger the condition. Certain bacteria have also been suggested as a cause of CFS in some people, including types of bacteria responsible for pneumonia.
However, while tiredness is normal after a viral infection, this doesn't explain why symptoms persist and get worse in CFS. Also, many cases of CFS don't start after an infection, and this theory doesn't explain why the condition sometimes develops gradually.