This article covers the zombies in the Rot and Ruin books by Jonathan Maberry.
This article uses an in-universe perspective.
The Reaper PlagueEdit
The cause of the zombie plague is a bio-weapon called Reaper. It is a mix of 9 viruses, 14 (15 in the mutated form) bacteria and 5 parasites, the main one called the jewel wasp. It started when a sample of a bio-weapon called Lucifer 113 got matched with another called seif al din, a Middle Eastern bio-weapon. (Seif Al Din is a prion that causes reanimation in the book Patient Zero, the first book in a related series, also written by Jonathan Maberry) It was further improved as scientists made a new version to make a protocol if something like it got out. Somehow, a group of people began seeding the compounds of Reaper 10 years before First Night. The CDC and others found the compounds 2 years before the dead rose.
Many signs of infection have been mentioned throughout the series. To get infected, you need to be bit by a zombie/other infected person (late cases will attack living creatures) some signs include:
Nausea, Aching joints, Double vision, Fever, Black veins coming from bite wound, Red blood vessels when eyes closed (Inflammation), Pins and Needles/Cannot feel legs
Symptoms can begin nearly immediately after infection, or can take about a day. Death and re-animation (ANYONE who dies without brain/spine injury will re-animate, although some bodies are immune to re-animation.) Can occur at 3 to 8 days after infection.