Is Chronic Fatigue Syndrome caused by anything but psychological factors?

A friend was recently diagnosed with Chronic Fatigue Syndrome. I realize that it is a real condition that is recognized by the World Health Organization. However, I was wondering if it is a psychological syndrome that causes real health concerns, or if it is caused by something more “tangible” (for lack of a better word) like a virus etc. The Center for Disease Control website indicates that a malfunctioning immune system can lead to CFS. However, can’t psychological disorders cause the same symptoms as CFS? Why/How is CFS not a psychological disorder? I am not trying to cause an argument; as I said, a friend was recently diagnosed and I am interested in gaining a real understanding of the syndrome. Perhaps a good book that explains CFS in a non-biased manner, or something by a medical researcher (not a psychologist) that explains the syndrome.