Puumala hantavirus (PUUV) is a zoonotic virus that in humans causes nephropathia epidemica (NE) in humans, a mild form of haemorrhagic fever with renal syndrome. An average of 10 000 cases are reported annually in Europe, many of which occur in Fennoscandia. The incidence of NE is connected to the distribution and population density of the the bank vole (Myodes glareolus), the main virus host. In Fennoscandia, high incidences of NE occur at 3-4 year intervals due to the characteristic population cycles of this woodland rodent.