This is based on opinion. Please don't list it on a work's trope example list.
Nightmare Fuel / Cabin Fever
A lot of stuff in this movie is going to seriously Squick out germophobes with the high Body Horror quotient inherent in the premise. The best part? Necrotizing fasciitis is absolutely real. Sweet dreams, kids.
It's actually implied to be STDs or syphilis in the first film, which causes ulcerated sores to melt your face off just because you went bareback. However, it's less contagious.
Paul Mercy Killing Karen with a shovel, for one thing, Karen's face which has most of her flesh gone and the fact it's implied Paul suffered a Sanity Slippage considering he saw Marcy ripped apart by the infected dog.
The Nightmare Sequence Paul suffers when he dreams of Karen before the infection only to almost kiss her when she looked like the aforementioned scene. Complete with a chilling Scare Chord.
The first movie shows why you should never shave with a flesh eating virus. Graphically.
Even the opening credits are unnerving, show what looks like a white sheet that becomes dirty over time before becoming bloodied, complete with an eerie tune with the ambiance of flies swarming nonstop.
The infamous, Fan Disservice scene in Patient Zero. Two hot babes duking it out on the beach? Hot! Two hot babes both of which are in the later stages of a flesh eating virus tearing each other apart on the beach, culminating in one caving the other's face in? Far, far from anything resembling hot.
Patient Zero qualifies for the series' mandatory sex scene horrifically. Namely, by showing exactly why you should open your eyes during oral.