I was reading articles for The Daily Collegian's freshman magazine (Fresh Start), and while most of the advice was sound, I think students do need to know one thing: Most professors want you to do well and there is room for explanations (not that the invisible dog ate your homework), especially if you get sick and miss class.
The professor won't automatically fail you just because "it's not high school."
I got pneumonia my second semester as a freshman and all of my professors -- I was taking 19 credits at the time -- were willing to work around my illness, either counting assignments I did before I got sick more heavily or letting me redo the assignments with extra help. Professors are not out to fail you. They don't sit down at their coffee table at home and say, "How can I confuse those new freshmen so that they ultimately fail and know the hardships of college life?" One of my professors told me to go home and not worry about class for a few days when I got sick, then spent three hours outside of class to teach me the material himself.
Get to know your professors. Walk in during their office hours and show them that you really care about the class. Just 10 minutes of your time might make up for the two hours of their time you were in bed with the flu.