It started with the disaster at the mansion lab:

Umbrella Corporation developed the T-Virus, a toxin for use in biological weapons. After breaking loose the virus turned all living things into hideous mutations. The experiment was a failure. The case was eventually closed but Umbrellas activities were far from over. A new virus has been released.

Raccoon City is now infested and bloodthirsty zombies overwhelm the community.

The nightmare has only just begun.