Simply replace the unfilled areas such as "" and "[date]" with your information.
Dear [name of teacher or nurse],
As you know, [name of child] has been absent from school for [days absent] days now. I have enclosed the physician's note excusing [name of child] from school, as well as the note stating that [he/she] is now medically cleared to return to school.
Although [name of child] did have [medical issue/illness], [he/she] is now no longer contagious, and will not be able to pass anything along to other children.
If [name of child] has fallen behind in any of the required class work, please let me know immediately so that I can assist [him/her] in catching up.