Last year, a Nevada team linked a peculiar retrovirus to chronic fatigue syndrome (CFS), an elusive condition with no known cause. The virus—known as xenotropic murine leukemia virus–related virus, or XMRV—had previously been implicated in an aggressive form of prostate cancer, but XMRV's role in both diseases has been hotly contested, particularly with regard to CFS.

Several research groups have failed to reproduce the initial finding, including a team from the US Centers for Disease Control and Prevention (CDC), but scientists at the National Institutes of Health (NIH) and Food and Drug Administration (FDA) reportedly have unpublished data supporting the link between the mysterious virus and CFS. “When the data are published, they will provide a confirmation of our initial discovery,” says Judy Mikovits, director of research at the Whittemore Peterson Institute in Reno, Nevada who led the research.

Spurred on by these mixed results, the FDA's blood products advisory committee met for two days in late July to discuss whether the virus poses a safety threat to the blood supply. Here are the facts they had to consider:

October 2009

8 October: Mikovits and her colleagues report traces of XMRV DNA in the white blood cells of 67% of a group of 101 people with CFS, compared with only 4% of healthy controls (Science 326, 585–589, 2009).

January–February 2010

Three independent research teams—two from the UK and one from the Netherlands—failed to detect significantly elevated levels of XMRV genetic material in their own samples from a total of 388 people with CFS (PLoS One 5, e8519, 2010; Retrovirology 7, 10, 2010; BMJ 340, c1018, 2010).

April 2010

6 April: Canada becomes the first country to recommend that individuals with CFS should not donate blood or blood products for fear of passing on the suspected virus.

May 2010

14 May: The Dutch team and two more research groups from the UK and Australia challenge the original paper’s methodology in a series of technical papers in Science (328, 825, 2010).

26 May: The NIH’s Harvey Alter allegedly announced at a closed meeting in Zagreb, Crotia that NIH and FDA scientists had independently confirmed the link between XMRV and CFS. In his PowerPoint presentation, Alter wrote that the data in the 2009 Science paper “are extremely strong and likely true, despite the controversy,” according to the Dutch nutrition and food supplement magazine Ortho.

June 2010

18 June: The US American Association of Blood Banks joins Canadian, Australian and New Zealand officials in recommending against blood donation from people with CFS.

July 2010

1 July: A team from the CDC fail to detect XMRV in blood specimens from 51 individuals with CFS and 56 healthy people (Retrovirology 7, 57, 2010). Meanwhile, a paper reportedly accepted in the Proceedings of the National Academy of Sciences USA describing the NIH-FDA findings is delayed for publication while the researchers conduct additional experiments.