I wanted to have the dead rise, and the infected turn. But some random person that locks him/her self away in a bunker shouldn't turn if they're not exposed in some way. So the infection due to exposure was my answer.
MW, Did you not like the ending?
The only other way I was going was to have Gun-Ho get offed and LJ and Repeater escape. And I wasn't ready to lose Gung-Ho just yet (no matter how much my son hates him because of his accent. LOL)