Iconian, a lot of games do not have logic behind them. They just need a story behind them. I do not see people like you complaining about radiated zombies from post-nuclear wars, even though, according to your logic, the nukes would burn and turn everything into ashes, therefore, will NOT be zombies. It's also impossible that Aliens, who you don't even know what biological structure they're made of, to be alive even after being blown up by the combustion of grenades.
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