Sick and ill are words that are similar in meaning but have clear differences in usage.
Editor Ben Korzec writes about these differences:
Sick is the less formal of the two words. It usually describes short-term diseases or ailments, like the flu, and is commonly used to refer to a feeling of nausea. Think of a sick child that stays home instead of going to school or a person who feels sick to their stomach after a roller coaster ride.
Ill is more formal and is used to describe long- and short-term diseases or ailments. A person who is not in good health because of something serious like cancer or pneumonia might be said to be ill. On the other hand, a person who is affected with a minor fever or cold can also be called ill. Like sick, ill also refers to a feeling of nausea.
Two common combinations of ill are "ill health" and "mentally ill." Ill is the preferred word in such cases, though sick can be used in its place.
