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Various ways.

1. Aliens make them rise up, so they save themselves the trouble of having to fight us themselves.

2. A virus gets brought back from Venus, or maybe a weird radiation.

3. The U.S. Army leaves containers of toxic zombie creating waste near high schools, usually in a building that is just perfect for underage drinking and teen highjinks.

4. A very amoral company called The Umbrella Corporation accidentally releases a virus, which in spite of blowing up Raccoon City is not contained.

5. Animal Rights fools let rage-infected animals out to bite us.

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15y ago

What else can I help you with?