(CNN) Chronic fatigue syndrome, a condition that causes extreme tiredness, could be triggered by an overactive immune system, a study has found.

UK researchers found that an exaggerated immune response can trigger long-lasting fatigue, suggesting this is how the condition -- also known as myalgic encephalomyelitis or ME -- begins.

Limited insight to date has made treatment a challenge.

Pariante explained that in a lot of chronic fatigue cases, patients remember an infection, such as a very bad cold or other viral infection, in the early stages of developing the condition.

"We had this information for quite a long time but didn't know what was going on in the body of these patients," he said.

836,000 to 2.5 million Americans struggle with the syndrome, according to the Around 250,000 people in the UK and 17 million people worldwide are affected by chronic fatigue, according to British charity Action on ME . An estimated836,000 to 2.5 million Americans struggle with the syndrome, according to the CDC

To try to find out more about it, the team modeled a possible route to the condition based on a treatment for chronic hepatitis C infections called interpheron alpha, because the treatment is known to induce persistent fatigue in some people.

Interpheron alpha affects the immune system in a similar way to a strong infection, the study explains.

Researchers measured fatigue and immune activity in 55 patients receiving hepatitis C treatment with interferon-alpha.

Patients were followed up before, during and after the hepatitis C therapy. Of the participants, 18 patients -- about a third -- developed lasting fatigue, defined as fatigue lasting longer than six months after treatment.

The group with lasting fatigue also experienced a greater immune response -- shown by a doubling in the levels of immune system messenger molecules interleukin-10 and interleukin-6.

Immune markers were measured with a blood test during the study.

The team also saw higher levels of these molecules in these patients before treatment had even begun.

"What this data strongly suggest is that people that develop CFS in response to an infection, they do so because their immune system is primed to hyper react," Pariante explained. Why a person's immune system could be primed to hyper react is unclear, but genetics could be a possible reason, he added.

Delayed diagnoses

Patients are often diagnosed with chronic fatigue five, six or even up to 10 years later, according to Pariante, because the diagnosis is based entirely on symptoms, such as headaches, difficulty concentrating and the inability to engage in activities.

By the time they are diagnosed, patients would have been ill for quite some time, making studies into the biology of people who develop the illness difficult, he added.

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Dr. Charles Shepherd, medical adviser to the ME Association in the UK, explained in an email that "many doctors still don't know how to diagnose and manage ME/CFS."

Shepherd, who was not involved in the study, said that a lack of research means "that we still don't have any effective forms of treatment."

Many people who have chronic fatigue find pacing helpful in managing their condition, according to the UK's National Health Service. Pacing requires people to balance rest and activity in order to avoid making their fatigue and other symptoms worse.

Shepherd said that the study "adds to the growing weight of scientific evidence which indicates that the body's immune system is playing an important role in the causation of ME/CFS."

The model the team used for the study "is relevant to the two most common causes" of chronic fatigue, Pariante said: a strong infection and a major stressful life event.

The study also found that "having quite severe acute fatigue during an infection, or shortly after infection" makes people more at risk of developing chronic fatigue, he added.

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Pariante hopes that once questions about why this happens are understood, future treatments and screening can be identified.

The study's small sample size was cited as a limitation by Pariante. More studies are needed to confirm its relevance to chronic fatigue patients, the paper says.

'Blackness descending'

Skye Wheeler, a 19-year old from Cambourne, Cornwall, is housebound due to her chronic fatigue syndrome.

"You can't get dressed and ready for the day without a blackness descending over your eyes. You can't carry a conversation, let alone even speak," she explained in a statement.

"The bottom of the line is that there is no energy there in the first place to channel through your body to all the right places. The only thing you can do is lie and exist," she said.

The condition affects many body systems, and people who have chronic fatigue syndrome feel debilitating pain and fatigue because their body and brain are unable to recover after even small efforts.

About a quarter of people affected by the condition are severe cases, meaning they are bedridden, house-bound or wheelchair-bound at some point in their illness, according to the ME Association.