"malady, sickness, ailment"
Ill arrived in the language, a borrowing from Old Norse, around 1200, but it was another 300 years before the noun is recorded. Throughout this time, the only sense for the adjective was "bad" or "wicked", with the noun expressing the related notion of "wickedness" or "evil conduct". By Shakespeare's time, illness was developing a wider range of meanings, such as "unpleasantness, disagreeableness", and this is how it is used when Lady Macbeth accuses Macbeth: "Thou wouldst be great ... but without The illness should attend it" (Macbeth I.v.18). She does not mean that, in order to become king, he needs to be unwell. The "disease" sense of the word, the only modern one, does not emerge in the language until towards the end of the 17th century.
David Crystal is author, with Ben Crystal, of Shakespeare's Words, published by Penguin