A new study found evidence that that chronic fatigue syndrome is not just "all in your head" but might be caused by a poor immune system.

Chronic fatigue syndrome (CFS) is a disorder characterized by extreme fatigue which doesn't go away even after a long rest. People suffering from this condition also experience fever, aching, and prolonged tiredness and depression that usually last for at least six months. The cause is not yet determined, but it usually starts after a viral infection.

Researchers at the Center for Infection and Immunity at Columbia University's Mailman School of Public Health found distinct immune changes in CFS patients that could explain that it is not a psychological condition, as what scientists previously thought.

The study involved 298 people diagnosed with CFS and 348 healthy people as the control group. The researchers collected blood samples from the participants and examined them for biomarkers.

The analysis found that those suffering from chronic fatigue for three years or less had 51 common immune biomarkers, characterized by an increase of cytokines in their blood. Cytokines are proteins that regulate various inflammatory responses, and having many of them is an indication that the immune system is being compromised. It could also mean that the patient's immune system didn't really recover from the viral infection.

"We now have evidence confirming what millions of people with this disease already know, that ME/CFS isn't psychological," lead author Mady Hornig, director of translational research at the Center for Infection and Immunity and associate professor of Epidemiology at Columbia's Mailman School, said in a news release. "Our results should accelerate the process of establishing the diagnosis after individuals first fall ill as well as discovery of new treatment strategies focusing on these early blood markers."

The researchers plan to continue the study to determine how the cytokine levels differ within patients depending on how long they have been suffering from chronic fatigue.

The study was published in the Feb. 27 issue of Science.