Dear Mr/Ms xxx;
Today I'm writing to you to ask for a five-day leave，since I've got cold last night with careless.
This morning my mother took me to see the doctor，who told me to stay in bed for some days. So I am very sorry to be absent from school，especially your interesting lessons. I'll be sure to make up for the missed lessons after I recover from the illness.
Yours sincerely，
xxx.