OK, I know I've talked about it a couple of times in the comments section, but I figured I should just make a thread to go a little more in depht about it.

Fist of all, I'd like to point out that my japanese is very basic, and while a machine translation at least gives you an idea of what's being said, it's still not perfect. This blog's reviews do a much better job of summarizing the games than I ever could.

Kansen:
gamesforadultsandeveryone...for-adults-halloween.html

Kansen 2:
gamesforadultsandeveryone...-adults-halloween_27.html

Kansen 3:
gamesforadultsandeveryone...-adults-halloween_28.html

Kansen 4:
gamesforadultsandeveryone...-adults-kansen-4-day.html

Kansen 5:
gamesforadultsandeveryone...for-adults-halloween.html

Kansen Ball Buster:
gamesforadultsandeveryone...-adults-halloween_10.html

So yeah, at it's core it's a zombie apocalypse series except the "zombies" are sex crazed maniacs, still the games use a lot of the standard zombie apocalypse tropes. At first the infectees still retain their old memories and are still capable of speech (although they tend to slur their words), however as the infection worsens, their minds deteriorate and they start acting like standard zombies. Also, while their main priority is sex, they do in fact eat other people, although this is never shown on screen.

From what I understand, the infection makes people bisexual but the games never show men getting raped by other men, with the exception of the second game, which has a few yaoi scenes with a trap. Usually getting caught by a male infectee simply cuts to the game over screen, while getting caught by a female one will show a brief sex scene before the game over screen.

Since the virus spreads through sexual contact this is a rare example of an eroge where the protagonist is trying to AVOID having sex. While there's still a few consentual scenes between the protagonist and the potential love interests (usually in the good endings), most of the sex scenes are focused on infectees raping people.

For me, the main appeal is the corruption angle, such as seeing the sweet, shy girls or the cool, collected girls get turned into sex-crazed maniacs is pretty hot.

There's a pretty memorable bad end in the third game where Ren (the hero's stepsister, the standard shy girl with glasses) goes yandere, after getting infected she comes across the protagonist and the other 2 main girls, who have barricaded themselves behind a metal gate in the sewers, Ren asks them to let her in. The protagonist is reaching for the gate, but the girls, who can tell from Ren's speech patterns that she's infected, stop him and inform him of her infection, telling him to just forget about her and move on. Ren overhears them and accuses the girls of trying to steal the protagonist away from her, at this point she completely loses it and goes berserk, banging at the door while screaming that she'll kill anyone who tries to steal her brother away from her. While the group moves on and goes deeper into the sewers, Ren and a group of infectees manage to get in as well and attack the group, Ren shoves the protagonist to the ground and has her way with him, resulting in a game over.

I know that the reviewer complained about the ridiculous infectee portraits, and I agree that some of them (the male infectees in particular) do look pretty goofy, but this was another one of my favorite parts of the games.
s.vndb.org/sf/44/10444.jpg
s.vndb.org/sf/34/44934.jpg
s.vndb.org/sf/32/44932.jpg
Yeah, they tend to vary between "silly" and "downright terrifying" but I still find it pretty hot due to the corruption aspect. In fact, one of my main complaints about the fifth game is that you rarely see the main girl's infected portraits, I specifically remember that you only see Satsuki infected briefly at the very end of one of the other girl's route. Kansen 5 in general had very few scenes of the main girls getting infected and assaulting the protagonist, a lot of the time they'd just disappear from the story after getting infected.

While all of the games except the fourth one got animated, I wasn't too fond of most of the OAVs. I don't think they portrayed the infectees as well as the games did, and most of them removed several characters.
The Kansen 5 OAVs were particularly bad, they removed 2 of the 4 main girls, yet they focused a lot on one of the side characters, hell they didn't even bother making the infectees' eyes red!
The Kansen 3 OAVs at least followed the game more closely, and had much better production values than the others.

Anyway, I just realized I typed in a lot more than I had planned, if anyone wants to discuss this or ask me something, feel free. (God, I'm lonely).

Ever seen David Cronenberg's Shivers? Sounds kinda similar

Huh, I had never heard of that before, but you're right.