Dear Mr. Abc,
I am writing to you to inform to you that I may not be able to attend office for a week from _________ to ___________, as I am suffering from disease. Therefore, it has been advised by the doctor to take of bed rest for faster recovery. I regret over the inconvenience caused to the firm because of my illness.
I am willing to put in extra efforts to compensate the delay at work as I am back from my sick leave.
Thanks and regards,