Day Zero.

During the 22nd Century, science and technology had advanced beyond ethical bounds, so most of mankind reversed course. Eventually, global dictatorships violated the United Nation's Bioethics Treaty, secretly stockpiling arms. As a result, an oblivious America was attacked. Retaliation with augmented nuclear weaponry had unforeseen effects, affecting most survivors in the Western Hemisphere and altering the rest. Those infected became ravenous beasts, feeding off the remaining population. But pockets of humanity remain.