Chronic fatigue syndrome (CFS) is a disease that afflicts people with extreme and debilitating tiredness that lasts for many years and isn’t relieved by rest. Some estimates suggest that it affects up to 1% of the world’s population. We don’t know what causes it. Prostate cancer is one of the most common cancers in the world and kills around 221,000 people every year. Its causes too are largely unknown. What do these two diseases have in common? They have both been recently linked to a virus called XMRV (or xenotropic MulV-related virus in full).