Endless Road of Corpse

The Ebola virus continues to wreak havoc, three Airlines airliners fall, and oil prices suddenly collapse. The zombie crisis suddenly erupted after a series of strange changes. 
Not only did the protagonist and his companions have to avoid the direct threat from zombies, struggling to survive in the apocalypse of modern civilization, but they also had to constantly search for the origin of zombies. Was this the greatest conspiracy in the history of human society, or was this a disaster that Earth was destined to face?