Because some went under and hidden themselfs from being exposed to the virus , also their were some which had immunity from the virus but since the virus spread too fast their were no time to develop a cure
Oh & due to demon contracts maybe it had something too not sure if the experiment was of demon contracts was under ground or even before the virus started but maybe have something to do with that too
But the virus was so over after 4 yrs right? I'm also confused.
Well the cause of the virus was the experments that they did on kids , once the virus started many hide in places the virus didn't reach not sure if some were already immune other than the ones under 13 , but seems the virus didn't spread everywhere this is why their were still human orgnazations not only in japan but in other countries as well , the virus was more like storm passed by also the ones younger than 13 became immune since the virus hit them already when they were under 13 so more like natural immunity from the virus when they grow up ..
Okay the humans above 13 which survived the virus was because cursed gears >>> http://www.mangago.me/read-manga/owari_no_seraph/bt/305831/Ch23/22/
For the ones got hit by the virus and younger than 13 developed immunity can't remember which chapter exactly but the virus can't effect them anymore
So in Owari no Seraph there is a virus that affects people above the age of 13. 4 years after the virus there are some kids who are above the age of 13. Is it because the virus doesn't affect people (of all ages) anymore? I'm so confused! （￣へ￣）