Appendix Figure. Human cases of hemorrhagic fever with renal syndrome (HFRS) per month from HFRS-endemic Västerbotten County, Sweden, July 2004 through June 2008, and measured snow depth at 3 locations through February 2008. The season 2004–05 represents the most recent epidemic peak year, before the large outbreak of 2006–07; 2005–06 represents an ordinary low-incidence season. The exceptional increase of HFRS cases in midwinter 2006–07 followed a rapid snowmelt and complete loss of protective snow cover to the voles during December 2006 in inland and coastal areas. Similarly, the less pronounced increase in midwinter 2007–08 followed a less abundant loss of snow cover only in the coastal area.