Damn there's a jump.
@Adalbertus123
At least two things. First, "our world" doesn't necessarily mean "The World" as in all of the planet Earth, it can also refer to something more localized but still common among a group. So "our world" could mean "advanced Japanese civilization" or something like that.
Alternatively if it did get out, it was established a few chapters ago that there can be an incubation period, it's not like infection always = instantaneous symptoms. If the government wasn't being careful, a few hours would be plenty for someone to get to at least Asia in an aircraft. And even if they suddenly went Ant on the plane, if nobody knew what it was they'd probably be restrained and the plane still might land in another country. After that it'd be one accident away from spread.
Of course, none of that even assumes maliciousness. If anyone
purposefully wanted to infect other places and had some level of resources/dedication it'd be pretty trivial early on by plane or boat. While there are of course luxury ones going for however much anyone could possibly spend, older motorized boats (ignoring sailing) capable of a comfortable ocean crossing can be had for a few hundred grand and would have plenty of space to contain a number of infected for a trip of a 3 weeks. Of course at some point I'd expect advanced countries to start using their navies and coast guards but they might not realize the threat in time, or someone could just go elsewhere first.