*double post* using my vjta

the real question of resident evil is how chris,jill,rebecca,barry,claire,leon,sherry,ada,etc. are all immune to whatever virus they encounter lol

Hypothetically speaking, if RE universe does exist, the virus has spread all has gone out of control beyond human means to contain it. Wesker made it all too possible, with the virus augmented him superhuman capabilities. Even in our world, we all know too well that majority of us is corruptible. This is even truer with Wesker, coupled with a virus.
We all know Wesker manipulates the storyline behind the scenes. It's the same way the most addictive drug affects people. He's corrupt, it revolves around power.

If you live in that world, it would be just fun to go out the balcony and shoot a zombie in the kneecap, chest or the face.

Like Silent Hill it adds to the atmosphere.

Let's do a recap of the games I've played.
RE1 some bullshit spec ops enter a mansion. No idea what's going on. Zombies. WTF. Croc gets mutated by the virus, plant gets mutated by the virus. Not much else really. Lots of puzzles, like the mansion is zombie HQ or something.
RE2, They're in the city. Don't really remember a lot of this one. I guess the idea was the outbreak spread from the mansion to the city.
RE3 (also known as greatest RE), new form of the virus creates Nemesis. He can open doors. AAAah! Tries to eliminate the stars members.
RE4 Some guy always trying to sell or buy things from me. Lots of big monsters. New viruses? There's another mansion at the end, but it's all a blur. (maybe I should replay these damn things)
RE:Code veronica: RE1 but with fast zombies.
RE5 African zombies, more crazy animal zombies, lots of the zombies for some reason pop up like parasites when you shoot off their heads. Since when did the virus become a parasite? I think this was in 4 as well, and possibly 3.
RE6 All kinds of abominations. Is this game even about zombies anymore?

tl;dr I'm excited for zombieU

I'm kinda fuzzy on the details, too.  I made a post remarkably similar to yours but I deleted it because I knew I had a lot of stuff wrong.  Anyway:

• RE0:  A virus that was supposed to help people gets used for evil.
• RE1-3:  An outbreak in a mansion spread to the city and the city was destroyed.  Umbrella crumbles
• RE4:  Terrorists have discovered a parasite that give them control of people but disfigure them in the process.  
• RE5:  Wesker returns with a new virus.  This one can only be used properly by certain worthy individuals.  Those it rejects become horrible monsters.  Those it accepts get super human power.  Sheva has big ol titties.
• Resident Evil Revelations:  An outside group (not Umbrella) releases a virus....I'm actually not sure what their motivation was.  They hated cities on the ocean, I guess.

As a Resident Evil nerd (who has actually come to resent the old RE games), I can explain this. Publicly, Umbrella was a huge corporation who made everything from pharmaceutical products, cosmetics, and even computer technology and it can be assumed that they had a lot of dealings with various governments throughout the world. Unknown to the public, the company was founded by certain people who had other goals in mind. One of these people, Spencer had the goal of more or less creating an Aryan Master Race, which is how we got Wesker. Aside from from that, the deadly viruses they made were eventually sold to the highest bidders in the underground black market of.......deadly bioweapon trading. This included terrorist organizations and rouge governments.

This plot ended up the way it did because the first few Resident Evil games were inspired by b-horror/sci-fi films and didn't have stories which were meant to be seriously (a secret bio lab disguised at a mansion in the woods, come one). At some point, it was decided that the series should have a serious story and a continuity that works, so Capcom went from treating it like a b-horror film to treating it like a Mission Impossible film directed by Michael Bay.

In reference to the bolded, I totally agree.  I couldn't count the number of times I finished Resident Evil 1--I had it down to a science.  The same goes for Resident Evil 2.  When I got to Code Veronica and RE3, I only finished them once.  I did get the two Gamecube games, RE Zero and the Remake of RE1 but by that time, it was getting pretty obvious that everything they could do with that style had been pretty much done.

Personally, I LOVE RE4, RE5, and Revelations.  They may not be scary.  Hell, they may not be what fans of the first games think a Resident Evil game should be.  One thing they are is VERY enjoyable.  I've probably finished RE4 more times than all of the old school Resident  Evils combined.  I keep going back to RE5 and Revelations every few months, too!