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New research has found myalgic encephalomyelitis/chronic fatigue syndrome (ME/CFS) is partly caused by genetics, related to the immune and nervous system.
The study - the world's largest into the condition - identifies DNA differences that raise the risk of developing ME/CFS and tells us about possible biological causes of the illness.
The study is being hailed as a milestone, but not a definitive test. Some of the eight differences are also often found in people without ME/CFS, so it cannot yet be cleanly separated who is at risk and who is not.
Three of the most likely genes produce proteins that respond to an infection. Another likely gene is related to chronic pain, but nothing has been found to explain why more women get ME/CFS than men.
Professor Chris Ponting from the University of Edinburgh, investigator on the DecodeME study, told RNZ's Saturday Morning much more was left to learn.
"The genetic link has been found between many diseases and explains quite a lot of why people have been so unwell for many other diseases, but for the last 15-20 years, when this technology has been available, it's not been applied to this, which is very female-biased.
"We felt that this was about time that a disease that is highly common, particularly among women, should have the benefit of the genetics, which also improves the chance in the future of getting an effective drug."
An estimated 67 million people were affected by ME/CFS.
The study looked at more than 21,000 ME/CFS patients, and concluded "both immunological and neurological processes are involved in the genetic risk" of the "common, poorly understood disease that has no effective treatments".
"About five out of six of our participants were females - it's a mystery why women are more likely to be diagnosed. We haven't found a good explanation for that."
Ponting said the research was starting to uncover what caused it in the first place.
"This is a disease that about two out of three people report after an infection, and for those individuals, they don't recover from that infection - and in that respect has some similarities to long Covid."
The key symptom was "post-exertion malaise".
"That is, essentially, the ability to redo something - so it might be physical, it might be mental and you might be able to do it really well on day one, but on day two, you try and do exactly the same thing and you can't. You've crashed in the intervening time, whereas those people who don't have ME are, you know, probably more likely to do better the second time round, so post-exertion malaise is a key difference.
"Lots of pain, unfortunately, loss of autonomic differences, people can feel quite faint, lots of sensitivities to light and sound, and to food, often.
"About one in four people are so badly affected that they're bedbound or housebound, and are - in this country anyway - shut away from life and it's an appalling disease."
Patients often report their doctors do not believe them, Ponting said - but the discovery of genetic markers should change that.
"One of the main achievements that I've been told by people with ME - because we've had such a fantastic response from people around the world - is that they are telling me that they now feel heard for a disease that friends, family and healthcare professionals not always have understood, and have not always accepted as being something that is a true disease.
"So they are telling us that this is a very welcome first step, despite - as you say - not immediately saying more about how to diagnose people or how to treat them effectively. This group of people is different in these eight different genetic places than the general population."
Combined with the knowledge gathered on long Covid in the past five years, Ponting hoped a cure could eventually be developed.
"It is a long journey that needs to be taken and our role in that is to fire the starting gun for the race to get an effective cure, and to accelerate that prospect, by making all of our data where participants have consented available.
"Our results have been downloaded by scientists from around the world dozens of times already. The individual data can be applied for.
"We're making it all available, so that all of those people around the world have the best chance of having a diagnosis and an effective cure within their lifetimes."
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