Scientists have discovered a potential retroviral link to chronic fatigue syndrome (CFS), a debilitating disease that is estimated to affect upwards of 17 million people worldwide.  Judy Mikovits, from the Whittemore Peterson Institute at University of Nevada-Reno (USA), and colleagues identified a newly identified retrovirus, XMRV, in the blood of 67% of CFS patients (as compared to 3.7% of healthy people participating in the study).  Not only did the blood cells of CFS patients contain XMRV, but they also expressed XMRV proteins at high levels and produced infectious viral particles. The XMRV retrovirus was first discovered in men who had a specific immune system defect that reduced their ability to fight viral infections, and has since been suggested to be a potential trigger of prostate cancer.  While this finding shows there is an association between XMRV and CFS, it does not prove that XMRV causes CFS.  Yet, the team observes that: "We now have evidence that a retrovirus named XMRV is frequently present in the blood of patients with CFS. This discovery could be a major step in the discovery of vital treatment options for millions of patients."