My theory is that it's like that one train from a mod from project zomboid. He will get the symptoms up until the point where other people would become infected and then begin to heal back to normal
The gun alone I don't think so as there are way too many students so they could overpower him and even if they don't want to take that risk they could steal the weapon while is is having sex or kill him in his sleep
I don't remember a lot of it tbh. Its about a brother and sister that get stuck (and separated) in another dimension in which they have to live in the roofs of buildings and everything and everyone wants to kill them. I didn't read a lot of this one but it's the OG story. It has a side manga...
I read a long time ago so I will say what I remember of the plot (it contains some heavy stuff) a zombie outbreak suddenly happens but it only affects men mutating them into monstrous forma which are far stronger. Of course it being a NSFW manga the zombies what to do a lot more than just eat...