The virus: prions, not zombies
An old DayZ intro, written by the game’s creator, explains the pathogen: it replaces a protein in the brain with prions, much like mad cow disease. Of those exposed, about 86% die, roughly 12% become chronically infected - alive, but with no normal brain function and an uncontrollable rage for blood - and around 2% are immune. That last 2% is you, the player, which is why you never turn when attacked.
DayZ is emphatic that these are not zombies. They are living human beings infected by a highly communicable virus, and because they are alive, they can be put down with body shots, not only headshots.
The wider world is infected too - Chernarus, Livonia, Sakhal and the upcoming Badlands region of Tachistan - yet the developers call it an epidemic, not a pandemic, implying that pockets of the globe still hold out. Routine Russian and American aircraft crashes, and the fact that Livonia’s only permanent gas zones are its airport and a treatment centre, hint that the infection travelled by plane.