The enemies. You never know what new mutation lays just around the corner and what monster-ous powers and abilities it possesses. It's also a somber-ing experience as many of these infected people/animals are innocent beings who wanted no part in the experiments/outbreaks they were caught up in and now mindlessly seek to to destroy/infect others. I actually feel for these monsters and want to end their pain rather then view them as obstacles that prevent me from moving forward. Excellent job on Capcom's part no doubt. The story while silly, is also a high point for me as is just about everything Resident Evil does; it just flows so nicely and keeps the player entertained.
Stupidity isn't a virus, but it sure is spreading like one!