Bites from mosquitoes carrying unidentified viruses might explain childhood leukaemia clusters around the town of Fallon in Nevada. And last week, a separate UK report found no link between nuclear power plants and childhood leukaemia.

The Nevada cluster is the largest in the US. Previous research failed to find a link between the cases and carcinogenic chemicals. The new study of the 14 Fallon cases that arose between 1997 and 2003 - a rate 12 times higher than normally expected in such a period - concludes that military personnel may have brought a virus to the area, which was then spread by mosquitoes. The cluster "fizzled out" once all vulnerable children had been infected.

"The rural location of most cases suggests mosquitoes as a possible vector," says study author Joe Wiemels of the University of California, San Francisco (Chemico-Biological Interactions, DOI: 10.1016/j.cbi.2011.02.019).

In the UK just 20 cases occurred within 5 kilometres of 13 nuclear power plants between 1969 and 2004 - six fewer cases than at seven "control" sites where nuclear plants were planned but never built. "We should be looking in other places for possible causes," says Alex Elliott of the Committee on Medical Aspects of Radiation in the Environment, which produced the report.