In my second semester of sophomore year, I was moderately sick for most of the semester. It was not life-threatening, but I often could not attend class because I didn't feel well. As a result, I was behind and had to work a lot to keep up. My research output was significantly lowered during this period also, for the same reasons.
However, it's not like my semester was bad - my GPA was lower than usual, but from a 4.0 to a 3.9. My research was lessened, but I still did some productive work. It's just that it turned a perhaps stellar semester (at least, a semester in line with the rest) to a perhaps good one. One slightly less great semester might not matter much for some grad schools, but I'm aiming for top ones as well. Is this something I could ask my advisors to note in their letters?