Everyone still alive right now is probably at least some degree of resistant, while people like Otis and the boss from the office were probably turning into Burned Ones even back before the fog fully rolled in, which is why they were out sick already. I figure the fog has two separate properties. One is that it's corrosive, which causes pain, slowing people down and making them vulnerable, and the other is that it carries some pathogen, possibly a fungus, that mutates people. Our MC and co are probably all at least resistant if not immune to the latter, but not the former since it's just a chemical reaction.
Of course, getting a full blast from a burned one might still be enough to overcome some degree of resistance while just some time outdoors can be recovered from. It'd explain where the first batch of burned ones/other infected came from (Otis and co) as well as why that guy who was in the locked office turned. He was already infected and turning before he locked himself in, then later broke the window when he started craving the fog.
At this point I kind of suspect the whole thing is an experiment/field test of a biological weapon by some Umbrella-like company and the military. They picked a dying small city in the off season to limit casualties while still getting to see how it performs in an urban environment. Anyone they do rescue can be used for longitudinal studies on the long term effects of different exposure patterns (ie, if they do use it in the field, and some friendlies get exposed, do we have to kill them all to make sure they don't turn into monsters years down the line, or are they good)
I also kind of expect that some people mutate into more of a spore-producer type of infected. Just big and stationary and pumping out the fog full time. Good body horror stuff.
