In the game deadrising, what is the cause of the zombies?

****spoiler alert****

Somehow, US government research created zombie causing wasps while trying to mass breed cattle in Santa Cabeza. Special Forces were called in, and all the zombies in Santa Cabeza were killed. Carlos, angry over how the people of Santa Cabeza were killed, somehow came across some of the zombie wasps and caused the outbreak in Willamette, Colorado. The game doesn't say if the method was from him bringing a wasp or an infected orphan who was given a delaying drug being sent there. The game has a few plot holes, but they can be covered up nicely with severed zombie parts.