JC virus particles, coloured transmission electron micrograph (TEM). This polyomavirus is found in 70 to 90 per cent of the population, where it usually lies dormant causing no symptoms. However, in individuals with a suppressed immune system the virus can reactivate and cause the fatal brain disease progressive multifocal leukoencephalopathy (PML). Magnification: x50000 when printed at 10 centimetres wide.