The words ill and sick are very similar in meaning but are used in slightly different ways. Ill is generally not used before a noun, and can be used in verbal expressions such as fall ill and be taken ill. Sick is often used before a noun.
In British English, ill is a slightly more polite, less direct word than sick. Sick often suggests the actual physical feeling of being ill, for example nausea or vomiting.
In American English, sick is often used where British people would say ill.
/Collins New Student's Dictionary - examples omitted