Sky of Ashes

She's awoken in the middle of the night, by her shaking and afraid mother, telling her she needs to run as far as she could as the world was "ending". An awful virus has broken out, extremely contagious and fatal, and a riot for survival. She meets a girl over the first week. But weeks turn into months, and months turn into a year. The girls have only spoken to each other for that whole time, barely seeing anybody else surviving and when they did, they were half dead or sick, or fending for themselves. They become best friends quickly and warm up immediately. but there's almost no explanation to where the virus had come from in the first place, or where everybody had gone to.