詞性相同
ill和sick均為形容詞，在（be, become, feel, look, seem）後作表語，表示“患病”或“身體不适”。
例1. John is ill and can't come to school. 約翰病了，不能來上學。
例2. Mary looks ill. What's wrong with her？瑪麗看起來病了。她怎麼了？
例3. Alice fell ill while on holiday. 愛麗絲度假時生病了。
例4. If you spend too much time in the cold, you will get ill. 如果你在寒冷中呆太久，會生病的。
例5. Joe felt sick and had to go home at lunchtime. 喬覺得不舒服，午飯時間不得不回家。
例6. I am sick in bed this morning, and I will report in sick. 今天早上我卧病在床，得請個病假。
詞義差異
在美式英語中，sick等于ill，但ill比sick更正式，尤其是指危害健康的嚴重的、慢性疾病。長期或短期的疾病，從輕微的發燒或感冒，到肺炎或癌症等嚴重疾病，都可用ill，有時也表示“惡心”，與sick意思相同。在英式英語中，sick意為“惡心”，一種可以引起嘔吐沖動的胃内不适感，而“生病”通常用ill，不用sick。
例6. I was ill for a time last month, but I'm fine now. 上個月我病了，現在好了。
例7. She's critically ill in hospital. 她住院了，病得很重。
例8. Is there a bathroom? I feel sick. 有洗手間嗎？我覺得惡心。
例9. If you eat that food, you'll be sick. 如果你吃了那種食物，你會嘔吐的。
例10. Joe ate so much that he was sick. 喬吃得太多，以至于吐了。
例11. The smell there made me sick. 那裡的氣味讓我惡心。
例12. I can't travel by boat because I feel sick immediately. 我不能坐船旅行，因為上船暈就想吐。
例13. Joe is very sick today. 喬今天病得很重。（美式英語）
例14. He has been sick over a year. 他病了一年多了。（美式英語）
例15. I have to call the doctor because my daughter is as sick as a dog. 我得打電話給醫生，因為我女兒病得很厲害。（美式英語）
修辭差異
ill和sick的修辭不同。sick用于修飾名詞（人），表示“病了”，ill不可以。Sick可以與day, leave和pay搭配使用，構成sick day病假，sick leave病假或sick pay病假工資，ill不可以。ill修飾名詞，表示“病了”通常要與副詞（seriously, chronically, or terminally）配合。
例16. John is ill, and he is an ill a sick child. 約翰病了，是個生病的孩子。
例17. Alice was at home looking after her ill sick baby. 愛麗絲在家照顧生病的孩子。
例18. She's not at work today. She’s on sick leave. 她今天不在上班，請病假了。
例19. This ward is for terminally ill patients. 這個房間是臨終病人的病房。
例20. In a clear, concise, straight forward style, they discuss the communication skills necessary to master the care of the seriously ill patient.他們以清晰、簡潔、直截了當的方式讨論掌握重症患者護理所需的溝通技巧。
例21. This is an innovative way to improve care for chronically ill patients.這是改善慢性病患者護理的創新方法。
ill和sick的比喻修辭法存在差異。ill 表示“不詳的；不好的；不愉快的”，sick表示“不愉快的”，尤指憤怒。
例22. Some believe that the owl is a bird of ill omen. 有人認為貓頭鷹是一種不祥的鳥。
例23. The report will induce an ill opinion of the new immigrants. 這份報告會引起人們對新移民的反感。
例24. There will probably always be an ill feeling between them.他們之間可能永遠都會有一種不愉快的感覺。
例25. Bill was forced to retire because of ill health. 比爾因身體不好而被迫退休。
例26. He makes me sick the way he behaves! 他的行為讓我惡心！
例27. I am so sick of all these ads. 我讨厭這些廣告。
例28. I got the feeling that he was getting pretty sick of me. 我有種感覺，他已經對我感到厭煩了。