Microbiology was actually my most difficult college course. (Calc being a close second)
Neither were actually that difficult, they just seemed that way due to where I was at the time. Calc was during my freshman year of college and I wasn't prepared to work hard. Microbiology was my first real biology course and I wasn't prepared to have to work as hard as I did. (I actually ended up with the best grade in our class in the Micro course - so I actually overprepared).
I think I attended an easy college and had relatively easy courses. I didn't take anything nearly as difficult as the courses most of you are taking.