If 3/4ths of the population died off, what do you think would be the best thing to do with the surviving population?

I am reading this YA novel called “The Big Empty” and I’m having trouble getting into it. One of the main reasons is that I don’t find the plot very plausible.

Basically, ¾th of the US population dies out due to a nasty virus. The US government collapses and martial law is declared. The middle of the US is gutted out and everyone is forced to live in the cities along the coast.

This seems like a really bad idea to me for a number of reasons. I’ll post them later—but I’d like to know your opinions on what would be the best way to sustain life if almost everyone were wiped out.