Same here

I like to imagine it had to do with his mental state. Your emotions and your physical state are biologically connected, so his intense stress, determination, grief, desperation and whatever else was going through his mind was directly influencing his physical transformation

Watsonian reason: the T-Virus (or whatever the film version is) induces randomized mutations as well as the necrotizing flesh. Bryan just happens to get an aggressive set of mutations. Doyalist reason: He left a note to his girlfriend, that guarantees he's at least an area boss.

Because he left a note. That made it so he would be a tragic boss the hero would fight. Making it to the top of the hospital made it so he would be one of the final bosses, if not the final boss.

its not specified which virus he has exactly but its likely because of how long it took to fully take over. viruses adapt and evolve. it also probably became more aggressive because of the proximity to the cure, thus mutating him in such a short time

Maybe that why the rolling flesh thing was chasing him all along just to mate with the alpha lol

He got exposed to what probably counted as a few strains. Touched blood on his car seat from the creature, got bit by the dog (likely? Somewhere? Even a scratch is a big deal), got bit by the kid, got that acid shit on his feet/shoulder, and ran through the sewers.

You're underestimating the chimera that was chasing him If it has the right keys for the metal net fence door, Bryan could have die there

One thing I can tell is that Bryan may be exposed directly to the BOWs, while zombies are not. So, in my theory, one of the scientists from umbrella has exposed themselves to that virus, costing the infection the early hours to RC

Amplification of the qualities of the host body?

@grok answer