Anyway, yeah that's what I was thinking too, like a slow buildup and then bam, fuck, zombies everywhere.
Genosha will be closed off and Magnus will hope for the best, tbh.
As for how widespread, I would say origin in New York and it could likely spread worldwide. I guess it just depends on what everyone thinks. It could be in North America and the rest of the world is like "fuck you guys" and quarantines it, but I could see it spreading regardless. All it would take is one person dying of an infected bite to get out of the country and start spreading it.