Near the end of the 15th century, people in England (and later Europe) suddenly began coming down with chills, headache, and pain around the limbs and shoulders. Within hours, they would break into intense episodes of sweating; within a day, as many as 50% of victims would die. The disease came to be known as sweating sickness.
Outbreaks of sweating sickness continued into the 1500s, showing up for weeks at a time in the late spring and summer. The last outbreak of the sweats was documented in 1578, after which it seemed to vanish altogether. Some theories for its cause include relapsing fever, a tickborne disease with some similar symptoms, and an unknown species of hantavirus—viruses typically spread through rats that are capable of quickly causing death.
Interestingly enough, mysterious outbreaks of sweating also showed up throughout France in the 18th and 19th century. These outbreaks were eventually blamed on a disease called Picardy sweat. But its described symptoms aren’t completely the same as the sweating sickness, and it’s not known if the two illnesses were related.