Could the Virus have been caused deliberately by someone? Spoilers
Spoilers for those who have not seen Resident Evil:The Final Chapter.
I was watching Resident Evil The Final Chapter and they explained that the T-Virus was released deliberately by the Umbrella Corporation to infect and destroy the entire world, just like how God flooded the Earth in The Holy Bible to destroy the wicked, Umbrella destroyed all of the rogue nations and powers to emerge victorious and rebuild the world after their goal was achieved, controlled Apocalypse.
The virus that causes people to become Walkers could easily have been artificially created by someone to kill off most of the worlds population and allow someone to emerge as the sole power on Earth, only they found out that there is no cure for it, except to destroy all of the Walkers and rule when they are gone, some group or government somewhere could easily be waiting to emerge and rule, but their cure did not work, now they are screwed.
Basically a cure that failed to work, or waiting for all of the Walkers to rot and then ruling when they are not active, Hawaii or Puerto Rico would be excellent waiting safe zones for this.
Just another theory, probably totally untrue, a controlled Apocalypse gone wrong, or maybe the muslims created it, genetic jihad against the infidels, would suck for everyone including them if it backfired.