I just worked on writing something very similar for a teaching residence program and I was diagnosed with cancer my freshman year of college and my grades were much lower then I wanted that year b/c of being sick. I talked about how even though I didn't do as well as I wanted to, being sick and continuing with school-not withdrawing- shows my perseverance, dedication,and all that other good stuff.

Let them see that yes you were/are sick, but being sick isn't the main thing- the main thing is how strong of a person you are and a hard worker.