This is always hard for me, choosing stuff, but I'm gonna go with my choice for classic survival horror: Extermination.
The voice-acting? Terrible. It's a survival horror game of the early PS2 era. However, if you're playing such a game for its dialogue, you're just asking for disappointment across the board. You can just ignore the melodramatic B-plot going on and focus on...this.
You are being flown to the Antarctic to investigate the loss of contact with an important scientific research center therein. Upon gaining entrance to the facility, you find that the place is a mess. Broken equipment, power losses, fire, and then something else... It looks like some kind of parasite. Well, you're armed. Easily taken care of. Investigation continues, and you start to uncover that a study of these things has been going on, and that while they are infectious, there are treatments FOR that infection. Good thing too, because a little while later, your partner and and old war buddy gets a bunch of these things latched onto him and every cell in his body undergoes a sudden and freakish mutation.
The facility has an infestation of strange parasites who infect people and generally deliver to you the threat of same, and to become a monster like your friend did just now. If you let it go too long, you become one of them. The whole facility is getting overrun by creatures formed by the infection of people, animals, and just generally the scenery. As you progress through the game, the monsters and the state of the facility itself gets worse, to the point where WALLS ARE FUCKING EATING PEOPLE! Yeah, apparently the walls grabbed people and grew basically through them molecularly.
Your main humanoid aggressors don't just get steadily worse, they can become SUDDENLY worse if they come in contact with water, because the infection is strengthed by heat and water. So, it walks over a puddle and gets bigger, maybe several times. It gets to the point where you are surrounded by flesh, probably worried about changing into a monster, and dreading the sound of another parasite dropping to the floor. The music lends a fair amount of creepiness as well, so while it is old now, the game IS good.