Wow. All the adults just suddenly died? That's definitely an epidemic.
It looks like the hunt for men is triggering Kou's past memories of Hime. Enough so, that she got distracted by them, and didn't react fast enough to the sudden earthquake.
The SRY really is needed. All those children with dead parents are suffering. Kou can't be selfish and deny herself, because she's doing this not for herself, but for her world.