Sweating sickness, also called English sweat or English sweating sickness, a disease of unknown cause that appeared in England as an epidemic on five occasions—in 1485, 1508, 1517, 1528, and 1551. It was confined to England, except in 1528–29, when it spread to the European continent, appearing in Hamburg and passing northward to Scandinavia and eastward to Lithuania, Poland, and Russia; the Netherlands also was involved, but with the exception of Calais (a seaport in northern France), the disease did not spread to France or Italy.

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Apart from the second outbreak, all the epidemics were severe, with a very high mortality rate. The disease was fully described by the physician John Caius, who was practicing in Shrewsbury in 1551 when an outbreak of the sweating sickness occurred. His account, A Boke or Counseill Against the Disease Commonly Called the Sweate, or Sweatyng Sicknesse (1552), is the main historical source of knowledge of the extraordinary disease.

British physician John Caius, author of A Boke or Counseill Against the Disease Commonly Called the Sweate, or Sweatyng Sicknesse (1552), the main historical source of knowledge of this disease. Courtesy of the National Library of Medicine, Bethesda, Maryland

The illness began with rigors, headache, giddiness, and severe prostration. After one to three hours, violent, drenching sweat came on, accompanied by severe headache, delirium, and rapid pulse. Death might occur from 3 to 18 hours after the first onset of symptoms; if the patient survived for 24 hours, recovery was usually complete. Occasionally there was a vesicular rash. Immunity was not conferred by an attack, and it was not unusual for patients to have several attacks. Each epidemic lasted for only a few weeks in any particular locality.

Since 1578 the only outbreaks of a disease resembling the English sweat have been those of the Picardy sweat, which occurred frequently in France between 1718 and 1861. In that illness, however, there was invariably a rash lasting for about a week, and the mortality rate was lower.

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It is difficult to know what the sweating sickness really was. Caius attributed it to dirt and filth. All the epidemics occurred in late spring or summer, so it may very well have been spread by insects. The disease seemed to be more severe among the rich than among the poor, and the young and healthy were frequent victims. It is unlikely to have been a form of influenza or typhus. One 20th-century writer identified it with relapsing fever, which is spread by lice and ticks and has many characteristics in common with sweating sickness. That explanation is certainly plausible. It is improbable that sweating sickness should appear as a well-defined disease and then vanish altogether, although such disappearances, while rare, are not unknown. Contemporary scholars have suggested that the illness was caused by hantavirus infection.