"Cabin Fever" is pretty much my favorite horror movie of all time. The movie came out in 2006 and featured one of the most realistic killers of all time: a virus.

YouTube

Said killer virus does result in some people killing each other, and at one point dogs recognizing a wounded human and taking advantage of the situation, but still here's a great original and GORY story.  The scene with the woman shaving her legs in the bathtub will give women nightmares for the rest of their lives.  The movie has some nice subtle humor including a racial component that pays off big time later in the film (at first you go "hey, that's uncalled for", then you find out more later).  I don't think the trailer really does the film justice, in fact it makes it look much older than it is.   This is a "5 Nightmare" film out of 5.  Check it out:

Here's the ultra weird "Pancakes" scene from the movie.