[quote=Eternal_Flame]
my nation have an isolated island far from my nation mainland, can it be where the plague spread?and for the Elthana population, should be returned to previous amount or lesser?
[/quote]

Sure. The plague didn't come from anywhere plot related, if that's what your wondering, it's just a random plague - but you can give it a story or whatever, I don't mind. 

Elthana' population can remain. You had them at 60,000, right? Leave them there.