Slapping the bag is a time-honored tradition. At tailgates and pre-games (and post-games for that matter), the inevitable sack of wine is passed around, evoking imagery of ancient Greece...Except everyone is wasted on rail liquor and only the girls are wearing gladiator sandals. But can you get mono from this seemingly innocuous tradition?
The short answer is yes: mononucleosis (AKA the kissing disease) is a virus transmitted through saliva, so you can get it from any activity where you swap spit with someone.
If you'd like to avoid a throat coated in white (a fun side-effect of some strains of mono called thrush–a yeast infection in your mouth!), weeks of sluggishness and an overworked liver and spleen (like they aren't taxed to breaking point already what with collegiate drinking habits), then stop slapping the bag.
In fact, stop doing anything where your mouth will come into contact with something that 200 other people have already licked. This includes beer pong (just drink from your own cup), ice luges, party chugs out of a handle of vodka and making out with grimy frat boys.
While mono isn't as contagious as, say, the common cold, you can still get it from sharing utensils and meals with someone. Does this mean you should never offer up a bite of your burrito bowl? Of course not, that makes you a monster. Or Joey Tribbiani. Just use a little common sense and give your friend a different fork.
And for god's sake...Stop slapping the bag. The Franzia hangover isn't worth it anyway.