Here's an example of an actual Leave Application Written By A Teacher To Her Principal:
Dear ma'am/sir,
I regret to inform you that I will be unable to fulfil my role as [insert job title] due to having contracted a highly contagious fever.
Having consulted my doctor, I foresee that I will be unavailable between [date that you expect to be out], although of course I will keep you informed of any changes to my condition.
Please don't hesitate to get in touch if you require more information.
Regards,
[Your name]