So according to the series, a terrible disease causes most of the population to become mindless zombies, similar to those back in the U.S
and is only left to a few survivors who, I supose are inmune, to survive the outbreak and get the army to rescue them. Sort of familiar isn't it?
So my guesses are that someone from the States carrying the disease moved to Japan, unkowingly spreading the infection further. The reason we dont see Special Infected is that the virus has just been freshly introduced and, being isolated from America, hasn't mutated yet. The infected from both series share some traits too.
And Inb4 TITS joke from Nick.