SCP-008 is a complex prion, or virus, from The SCP Foundation. It is known as the zombie plague because that is literally what it is. It is the source of zombies. Symptoms of infection with SCP-008 manifest no more than three hours after exposure.
There is strong evidence to suggest that SCP-008 itself did not form naturally on Earth, since variants of it would have displaced much of the environment. According to SCP-052, the epidemic will take place in 2092 and will be caused by an SCP-008 containment breach.