What is defined as 'sickness'?

[Voralphion]

Yes elves could be poisoned. Aredhel the sister of Turgon was killed by poison in the Silmarillion by Eol.

But in the evening Aredhel sickened, though the wound had seemed little, and she fell into the darkness, and in the night she died; for the point of the javelin was poisoned, though none knew it until too late.