We know what happened somewhat. It is covered in season 3 with "Beyond Our Walls". But after Ft. Irwin and Boulder became the last large safe havens in the US we know nothing. There were outbreaks in other countries (I don't remember which ones though) and given that Irwin lost contact with other countries we can infer that the world has gone to shit. (knowing full well that Kc wouldn't just excuse the fact that there were US naval ships stationed all across the world)