Chronic Fatigue Syndrome: 'Surrounded by uncertainty'
When it comes to controversy and heated debate, few illnesses come close to Chronic Fatigue Syndrome, also known as Myalgic Encephalomyelitis (ME).
There have been arguments about whether it exists, the cause, the best treatments and even the name itself.
These disagreements have reached the level of abuse and death threats.
The disease itself, however, remains poorly understood - or as one doctor put it: "The whole thing is surrounded by uncertainty".
Does it exist?
For a long time, the existence of CFS/ME was widely dismissed and labelled as "yuppie flu". That opinion has largely been reversed in the past decade.
In 2002, then chief medical officer Prof Sir Liam Donaldson described it as a "disease in the wilderness". He was presenting a report which said CFS/ME was a "genuine illness and imposes a substantial burden on the health of the UK population".
It is thought to affect about a quarter of a million people in the UK.
The main symptom is severe fatigue, made worse by exertion, which does not go away after resting. Muscle pains, headaches, memory problems and depression can also be involved. In some cases it can be completely debilitating, resulting in patients being unable to leave their beds.
Understanding of the disease is largely led by those symptoms. There is no test for CFS/ME, instead it is diagnosed by ruling out other conditions which might produce the same symptoms.
The underlying cause, or indeed causes, have been more elusive.
One of the issues in the field is that there is an emerging consensus that CFS/ME is not one illness.
Prof Stephen Holgate, chair of the Medical Research Council group on CFS/ME, told the BBC: "I think the problem with it is the term is used as a bit of a dustbin."
In children there are thought to be three sub-groups, and even more in adults, all given the label CFS/ME.
Prof Peter White, of Barts and the London School of Medicine and Density, said: "Most specialist doctors and scientists agree that it is more than one illness. It may be three to five separate illnesses.
"Like kidney failure, it has lots of different causes, but looks the same."
There was a brief moment of hope and optimism that a specific virus - XMRV - was the cause, however, that link has been largely discredited.
Viruses may have a role as a trigger for the illness, with many patients reporting that their symptoms started after infection.
Yet this has further levels of complication, as Prof White argues: "If glandular fever is a trigger but a patient has symptoms five years later, then it is no longer the Epstein-Barr virus, its something else."
While there have been suggestions that patients with CFS/ME have differences in their immune systems, pain perception and hormones, it is not known whether these are a cause or symptom of a chronic condition.
There has also been suspicion of a genetic or family element.
In Nature Reviews Neuroscience, Prof Holgate argued: "One is left with a strong sense that post-viral events are a common trigger of Chronic Fatigue Syndrome, but how they lead to chronic persistent disease remains unresolved."
The search for a cure
So far, a cure does not exist.
Prof Michael Sharpe, of the University of Oxford, said: "A pill that made you better would be great." However, he added, medicine fell well short of that: "The best shot right now is various forms of rehabilitation."
The PACE trial, which published results earlier this year, attempted to find out what the best therapies were.
It examined the use of both cognitive behavioural therapy, which alters the way people think and cope with their symptoms, and graded exercise therapy, a gradual and supervised increase in activity levels.
The trial suggested that patients using these therapies showed lower levels of fatigue and greater physical function.
Prof Holgate said: "Combining graded exercise therapy and cognitive behavioural therapy has undoubted benefit.
"Yes you can improve, but it mustn't be confused with the underlying cause."
He said these therapies might only be treating secondary symptoms.
However, the study has generated controversy with patients' groups and charities saying the findings were exaggerated and went against their own evidence. In Action for ME's 2008 survey of patient experiences, more than one in three said graded exercise therapy actually made them worse. They prefer a method known as "pacing" - or learning to live within limits - which the PACE trial concluded was not effective.
The ME Association's Dr Charles Shepherd said: "We consistently find pacing is the most effective. I'm not convinced graded exercise therapy is the answer to this disease, it is something more fundamental which cannot be reversed, an overly simplistic solution to an extremely complex problem."
Some people do, however, get much better.
Dr Ester Crawley, who specialises in children with the illness, said: "The prognosis for adults is poor, but for children it is really good, up to 94% get better."
She said the reason for this was unclear, but theories included "neuronal plasticity" - as children's brains are not fully developed, they can heal better - and "the adult lifestyle being predicated against recovery".
It is easier to adjust a child's lifestyle, such as doing two hours of school a day and gradually building it up, which is harder to do with adults who are in work.
A deeper understanding of the illness is desired by all involved. Delving into the sub-types of the condition may help in finding causes, which could also have implications for treating each sub-type.
One hope is that the new biobank of blood samples of CFS/ME patients being set up at London's Royal Free Hospital will help provide the answers.