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The Walcheren Campaign involved little fighting, but heavy losses from the sickness popularly dubbed “Walcheren Fever”. Although more than 4,000 British troops died during the expedition, only 106 died in combat; the survivors withdrew on 9 December. […] Along with the 4,000 men that had died during the campaign, almost 12,000 were still ill by February 1810 and many others remained permanently weakened. Those sent to the Peninsular War to join Wellington’s army caused a permanent doubling of the sick lists there.
As for ‘Look Ass Peeps’ (Lucas Pepys):
In 1794 Pepys was made physician-general to the army, and was president of an army medical board, on which it was his duty to nominate all the army physicians. When so many soldiers fell ill of fever at Walcheren, he was ordered to go there and report. As a consequence the board was abolished; but Pepys was granted a pension.