Zombotech

Located in the heart of San Fierro, one of the most populous city of San Andreas, the government-sponsored Zombotech Corporation was researching biological weapons in it's underground labs. The facility was heavily damaged during the earthquake that precluded and caused the crisis and a little known highly infectious agent escaped in the open.

They first infection signs soon appeared in the crowded cities of San Andreas: crazed humans trying to rip apart anything with a pulse. Nobody thought about them to be zombies at first, since the San Andreas was already in a mess, with rampant violence on the streets and panicked refugees gathering in the few government safe camps. There was not enough food, there was not enough gas; only bullets and distrust were plenty and this only helped the infection to spread.

When the officials finally acknowledged this is an epidemic, it was too late and too big to be stopped. They dubbed it the 'Incident' and decided to contain it and wait for it to consume itself.