There are several theories as to what causes Fibromyalgia, from viruses, to trauma (physical or psychological), or an inherited disorder etc, but there is no definitive answer yet.
Recently the Whittemore Peterson Institute in Reno reported in the Science Journel that a retrovirus (XMRV) has been discovered to be present in 95% of people they tested who had Fibro, compared to only 3 or 4% of healthy test subjects.
The virus (xenotropic murine leukemia virus-related virus) is from the same group of viruses that cause AIDS and a type of leukemia in people and was first identified in men suffering from prostrate cancer. XMRV affects the immune system and may cause a range of illnesses. Although, just being present in people with Fibro is not to say that the virus is the cause of the illness.
Also, this is not the first time that a virus has been linked to Fibro and unless further trials replicate the Institutes' findings, it is unlikely the true cause of this debilitating illness will be found any time soon.