How to explain bad grades due to mono on the common app?

The week before senior year started, I was diagnosed with mononucleosis by my doctor. This made me miss the first two weeks of school and really affected my performance throughout the rest of the semester. I’m not sure what my final grades are yet for this semester, but as of now I have 2 Cs, 1 D and 4 As and these most likely won’t change all that much as teachers finish up their grade-books. Mono can take months to fully recover from, and I know that these low grades do not reflect who I am as a student. I just don’t know how to explain this in a way that doesn’t sound like I’m making an excuse for slacking off. I am going to write a note in the “additional information” section of the common app explaining all of this. How do I do this in a way that will help my chances of getting into college?

I think I would ask my guidance counselor to handle that in her letter.