The act or process of causing disease or making someone sick; becoming diseased.
From disease as a verb + -ing (present participle suffix). While 'disease' as a verb is archaic, the gerund form appears in older medical and poetic texts.
Medieval herbalists believed certain plants worked by 'diseasing' evil humours in the body—poisoning the bad parts while leaving healthy tissue alone, which is oddly how some modern cancer drugs actually work.
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