sickness：较口语化，可与illness换用，但还可表示"恶心"。
disease ： 指具体的`疾病，且通常较严重，发病时间也较长。
complaint ： 现代英语中，既可指症状，也可指疾病。
例句：
1. Attitudes to mental illness have shifted in recent years.
最近几年对精神病的态度已有所改变。
2. He boasts of dodging military service by feigning illness.
他炫耀自己通过装病来逃避服兵役。
3. She was determined not to let the illness take hold again.
她决心不让疾病再次击垮自己。
4. Although her illness is serious, her condition is beginning to stabilize.
虽然她病得很重，但病情正开始趋于稳定。
5. The illness had a profound effect on his outlook.
这场病对他的人生观产生了深刻的影响。