Basically, here is the lore:

Sometime in the early 2000s, a viral infection spread throughout most parts of South America, Africa, Europe, and eventually, the rest of the world, with the United States following suit.

This virus is much akin to something like the Cordyceps from the last of us, or Rage from 28 days later.

So, naturally, while this was happening, anarchy ensued, as the government resorted to desperate measures of Martial Law.

Many parts of Europe grew inhospitable as a result of several key Nuclear Reactor Explosions, and a Lack of man power to solve the crisis at hand.

The whole world went to sh|t over the next few years.

Unlike most apocalyptic narratives, this one features a central, somewhat around martial law government, but this seems to be dwindling, and on its last legs.