The H1Z1 virus spread fast, like a super aggressive influenza, affecting most of the world's population. The infected suffered dementia, an unnatural aggressiveness, and an instinct for cannibalism. They became able to regenerate their wounds over time, unless their brain was destroyed or their bodies burnt. They turned into zombies.
A very small subset of the infected experienced an unexplainable variation in the H1Z1 symptoms. They acquired the ability to regenerate, but kept all their mental faculties and suffered none of the aggressive behavior. Nicknamed Z1s by the media, they came from every walk of life and soon found themselves falling into the role of heroes.