The Grey Plague serves as the plotline background of Zombies, Run! The "Grey Plague" is the colloquial name for the virus ZN1, which caused much of the population of the world to turn into zombies. Characters often say that someone has 'gone grey' when he or she has been turned into a zombie.
The virus is spread via bodily fluids (blood, saliva), most often via a bite, although it can also more rarely be contracted through scratches from contaminated material. Symptoms are flu-like, including coughing and fever leading to death and then resurrection as a zombie.
ZN1 has a 100% fatality rate which should have worked in favour of the researchers working to combat the disease, as under normal circumstances, it would have burnt itself out quickly. However, no-one had counted on the dead returning as zombies and being able to continue to pass on the disease.
The first known victim of ZN1, was Arthur Gurkhan, making him Patient 0.