The English sweating sickness caused five devastating epidemics between 1485 and 1551, England was hit hardest, but on one occasion also mainland Europe, with mortality rates between 30% and 50%. The Picardy sweat emerged about 150 years after the English sweat disappeared, in 1718, in France. It caused 196 localized outbreaks and apparently in its turn disappeared in 1861. Both diseases have been the subject of numerous attempts to define their origin, but so far all efforts were in vain. Although both diseases occurred in different time frames and were geographically not overlapping, a common denominator could be what we know today as hantavirus infections.
Source: Viruses journal 2014 Jan; 6(1): 151–171.
The number of mortalities is unclear, but in one outbreak alone (London 1845) 15,000 people died. Modern-day hantavirus infections (e.g. HPS or HFRS) have some similarities.