Chronic Fatigue Syndrome may be triggered by an out-of-control immune system which overreacts to an illness or emotional stress, a new study suggests.

CFS, also known as myalgic encephalomyelitis (ME), is a long-term illness, characterised by extreme tiredness, but the biology of the disease has remained a mystery.

Now researchers at Kings’ College London have discovered that some patients who were given drugs to ramp up their immune system to fight hepatitis C show similar symptoms to people suffering CFS.

Out of 55 patients studied, 18 developed lasting fatigue, suggesting that their boosted immune system had triggered long term changes in the body.

And crucially, even before treatment, those who went on to develop lasting fatigue already had higher levels of biomarkers associated with inflammation, suggesting their immune system had already been primed to over-respond.

Lead researcher Dr Alice Russell from the Institute of Psychiatry, Psychology & Neuroscience (IoPPN) at King’s said: “‘For the first time, we have shown that people who are prone to develop a CFS-like illness have an overactive immune system, both before and during a challenge to the immune system.

“Our findings suggest that people who have an exaggerated immune response to a trigger may be more at risk of developing CFS.”