sickness：較口語化，可與illness換用，但還可表示"惡心"。
disease ： 指具體的疾病，且通常較嚴重，發病時間也較長。
complaint ： 現代英語中，既可指癥狀，也可指疾病。
例句：
1. Attitudes to mental illness have shifted in recent years.
最近幾年對精神病的態度已有所改變。
2. He boasts of dodging military service by feigning illness.
他炫耀自己通過裝病來逃避服兵役。
3. She was determined not to let the illness take hold again.
她決心不讓疾病再次擊垮自己。
4. Although her illness is serious, her condition is beginning to stabilize.
雖然她病得很重，但病情正開始趨于穩定。
5. The illness had a profound effect on his outlook.
這場病對他的人生觀產生了深刻的影響。