Crossword Dictionary

ill, sick - a 
affected by an impairment of normal physical or mental function; "ill from the monotony of his suffering" 

ill, badly, poorly - r 
(`ill' is often used as a combining form) in a poor or improper or unsatisfactory manner; not well; "he was ill prepared"; "it ill befits a man to betray old friends"; "the car runs badly"; "he performed badly on the exam"; "the team played poorly"; "ill-fitting clothes"; "an ill-conceived plan" 

Examples:

He will not attend school tomorrow if he is still feeling ill in the morning.

If you are ill do not go to any social gatherings.