Jamieson, Fausset and Brown on Isaiah 33:24

24. sick--Smith thinks the allusion is to the beginning of the pestilence by which the Assyrians were destroyed, and which, while sparing the righteous, affected some within the city ("sinners in Zion"); it may have been the sickness that visited Hezekiah (Is 38:1-22). In the Jerusalem to come there shall be no "sickness," because there will be no "iniquity," it being forgiven (Psa 103:3). The latter clause of the verse contains the cause of the former (Mr 2:5-9).