suffering from or affected by disease, or figuratively morally or mentally corrupted.
From Old French 'desaise' (discomfort), from 'des-' (negation) + 'aise' (ease, comfort). Over time, 'disease' became the medical noun, and 'diseased' developed both literal and metaphorical meanings about contamination.
Medieval people used 'disease' to mean discomfort before it meant actual sickness—so describing a country's government as 'diseased' made sense because they thought of society as having a 'humoral balance.' The metaphor comes from real medical theory that we've forgotten about.
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