Since this word consists of a kanji with hiragana attached, you can bet that it will use the kun'yomi reading. You didn't learn that reading with this kanji, so here's a mnemonic to help you:
There is only one time you allow yourself to be sick, and that's when the Yankees (や) are playing. You can't miss any games so you just happen to fall ill whenever they're playing so you can stay home and watch. That's pretty convenient, isn't it?
Stress interviews make you feel really sick afterwards, don't you agree?
My coworker got sick from work-related stress.
I never thought that he would get mentally ill.