summary

I believe the North Koreans were experimenting with a new type of bio weapon.

In the movie noone knows if there is a cure, but they say there isn´t. Also, they say that the virus is dying with the host. No virus can survive if the host dies. I don´t know if it´s like that for real or only in the story.

There was no N. Korean bio-weapon.
If u watch carefully, u'll understand the virus comes from the woods, after a team went into the woods. When they returned, they were sick. They spread the virus through the whole guard post.
Everything started after that team came from the woods, so it is suggested that whatever happened to them there, infected them.

The virus makes you violent; however there r a lot of unanswered questions at the end of the movie.
It also alters your perception and u see things in a scary manner: u think u are attacked and so u attack to defend urself but in reality u're the one that starts the attack. Basically, u get scared of ppl and u r 'forced' to protect yourself by killing them. Except u're killing innocents. That's how i understood the virus' effects.