Annoyance.

This is the time of year when EVERYONE gets sick, and in a large lecture class, there's bound to be a handful of people coughing up a lung for the entire duration of the class. I understand their plight, as I've come to class sick before too, but I still hate it when I end up sitting next to a sick person because INEVITABLY, they will cough at the exact moment the professor says something important. Example:

Professor: ...Anyway, the thing you should really take away from all this is-
Sick person next to me: COUGHCOUGHCOUGHHURGGHNN
Professor: -and that is what your next exam will focus on.

...I'm sorry, professor, I didn't quite catch that. All I heard was my neighbor forcefully expelling phlegm from his lungs.

I hate flu season.