如果是小病，建議樓主說：Dear Mr. XXX, I wish you get well soon. Take care！
如果病的比較嚴重，住院的那種，就說：Dear Mr. XXX, I am sorry to hear that you are ill. I will keep my fingers crossed and wish you recover soon. Looking forward to your coming back to us healthily. Take care ! (親愛的XXX老師，聽到您生病我很遺憾。我會合十雙手住院妳早日康復。期待您盡快的健康的回到我們身邊。祝好！)