The causes of M.E. are still being investigated. Emerging evidence indicates that there are likely to be a number of factors involved and that there may be a number of different types or sub-groups of the illness.
There is evidence that certain infections can trigger the illness. Many are viruses but M.E. may be triggered in other ways.
Common viral triggers include glandular fever or Epstein-Barr virus (EBV). Other herpes viruses such as herpes simplex, VZV (causes chickenpox/shingles), HHV-6 and CMV, may sometimes trigger M.E., as can viral meningitis or labyrinthitis, commonly caused by enteroviruses, gastroenteritis, Hepatitis A, B and C infection, and in Australia, Ross River virus.
Non-viral triggers include toxoplasma, brucella, salmonella, tuberculosis, Q fever, and Lyme disease.
However, there is no clear evidence that M.E. is a form of persistent, chronic infection – it may be a consequence of infection.
There are likely to be a number of factors involved. It sometimes affects more than one family member. The reasons for this are being investigated but some studies indicate that genes can play a part, as can environmental influences.
It is not clear why some people get M.E. while others recover; some fully and others to a degree. Around 25% of people remain severely ill for many years. People who get M.E. may be vulnerable genetically, or their recovery after an infection could be affected by, for example, trying to return to work too soon, doing vigorous exercise, or experiencing major stresses.
Some people, including many researchers, believe that there are different illnesses being collated together under the umbrella term M.E. or M.E./CFS. This is because of the spectrum of severity, the range in collection of symptoms that individuals can experience, and the length of illness.
We know much less about other triggers, because they are less common. Some people may have had an infection, but didn’t notice it at the time.
In a very small number of people, the trigger may have been an immunisation, given perhaps when they were already unwell, or a physical trauma, such as a road accident, operation, radiotherapy or chemotherapy, or whilst rare, it is suspected that a very few cases may be triggered by certain toxic substances.
One big outstanding question is whether emotional stressors can be a trigger. Studies are not clear, some suggesting a link and others not. It is very unlikely that stressful life events, such as bereavement, can trigger M.E. on their own.
Often it isn’t possible to find out exactly what caused your illness – but you can still improve your symptoms, despite not knowing the exact trigger.
There is growing evidence from experts in the field of M.E. that a number of sub-groups exist within M.E., on the basis that individuals within these sub-groups differ in terms of their illness experience and the course their illness follows over time.
The likelihood of multiple sub-groups within M.E. most probably explains the huge variation observed by doctors in the progression of the illness and underlines the difficulty of making a prognosis. The experience of doctors specialising in M.E. is that some people recover completely (the rate is higher for young people) and tha others people improve, often significantly, over time. However, some remain very ill, often bedbound and/or housebound for many years.
Identification of sub-groups will, it is hoped, help doctors to personalise treatments and improve outcomes for people with M.E.