Dear ,
I am very sorry to inform you that I did not manage plete the book report you assigned last week, due to a sudden illness falling upon me a few days ago.
For the past few days I have been in hospital with a continuous fever, which has thus prevented me any academic activity. I hereby submit the doctor's note. I would be very much obliged if you could grant me another week for the task, as my health is turning better.
Hope you can understand my situation and accept my apology.
Sincerely yours,