Why there is dead people in Raccon City

Was replaying Resident Evil 3/2 and a question came to my mind, during a lot of scenarios you see people dead in the background but... why are they dead?, like the virus wont revive everyone and turn into zombies if they get bitten, "magically" like graveyard zombies in Code Veronica or even revive them like some zombies being eaten by other zombies and then wake up?

For example in Re1 the dude in the balcony is dead and then zombifies