So you may ask yourself how did it really come to an outbreak? Here is the full story, as posted by ex-head of development Braveheart.

The virus broke out on December 21st, 2012 - ZDay.

The outbreak started in Furor city. The government had always favoured war and wanted to be one step ahead of their enemies, so they built a secret research facility in the surrounding country-side of Furor City. This was a secret underground research facility which was completed in late 2007.

The virus research began in 2008, it took them a solid 4 years to come up with a deadly virus which would infect humans with rage and attack other human begins. This was a perfect way to infect enemy armies and have them kill and wipe out the entire country they so proudly fought for.

On December 21st, 2012, there was a power-failure in Furor City, the research lab was designed for safety, all locks were electronic and in the event of a power-failure all locks would open allowing the scientists a clear path outside, there were air-locks outside which scientists could close just in case any substances had escaped. Just before the failure, a test on the ZDay virus was being ran on some rats in an air-tight electronically locked room, the power-failure hit - the doors swung open - ZDay was here.

The virus infected the surrounding country-side and later reached Furor city, from there it spread through the country and by 4th January 2013, the world was completely infected, there are handfuls of survivors around the world; none of them had planned for such an event. The human race, is on the brink of extinction.
Of course, a cure was developed just in case they needed to for some reason reverse the infection, but it could only cure an early infection, once a human became a flesh eating human beating zombie - there was no going back.