How?

[Valentine]

Here's some questions: Moments before Wesker idead at the mansion the Tyrant impaled him with he's giant arm into his stomach leaving a big hole or at least somethng in dissorder. I suppose Wesker likes to eat when he's hungry so later Wesker was restored to normal HOW? Is the virus Regenerating?

What exactly did Krauser have in his body (I know it was Las Plagas somthing) Becauser the thing is as you all know he could get back from mutating to normal. Which all other RE bosses could not do.

 

[Kagara]

Yeah, Wesker's virus regenerated his stomach when he was "killed" by the Tyrant. When Wesker died the virus basically overhauled his whole body, reconstructing his muscles and whatnot to grant him super-human abilites.

As far as I know, Krauser had a Control Plaga. I don't think he could revert back to his Human form.

 

[Valentine]

Yeah you never saw him do that in the game other then in MERCS. But I think he could I don't know where I got that.

 

[nemesis_lover]

With Wesker regenerating, it's part of the T-Virus's power to make the virus carrier have super human strengh and power. The zombies didn't carry such power, but they were able to take multiple shots without much thought to pain or stopping.
Then again it's said that Birkin provided him the Virus, so i'm guessing it's something on the lines of a super T-Virus, but not the G-Virus. Wesker had planned to get killed by the Tyrant in order to activate the virus.
All this is thrown out of the window when RE5 came out, as we find out from Spencer, that there were more then one Wesker so to speak, and each one had the progenitor virus administered into each of them. I can't remember much of RE5, only completed it once fully, but I think he's the only one to survive. So he must of learnt that he needed to die in order to activate his hidden virus powers.
Also being killed in front of Jill/Chris made the news get back to Raccoon City, which then would of stopped Umbrella keeping tabs on Wesker, which would lead to Wesker having the freedom to join the other agency, which was white umbrella/Tricell at that time?
I could be very wrong in this information, but this is what i've gathered from a very confussing storyline lol. You can also replay Umbrealla Chronicles, they had a memo or two on Wesker and his death.

Regarding Krauser, I think him returning back to his human form, is just part of merchs, he didn't do it in the storyline for RE4, so i'm guessing it's just a one off transformation, as the rest of the bosses in RE4, seemed to have a one off transformation. I could be wrong on this matter though.