30 years ago, human civilization fell to the edge of destruction by an accident. Water, air, food... Nearly all essentials were polluted by a kind of unknown virus. Infected creatures lose their mind to attack others and make things worse as if they were controlled. The world lost 95% of its population in the last 30 years. But there is still hope for humanity.
About 10 years ago, a research of the vaccine succeeded. Unfortunately, the research facility in Block Z was invaded and fallen soon. To get the data of the vaccine, survivors keep trying to get into the closed area till today.​