From The Urban Dead Wiki
The precise origins of the zombie plague are unknown. News stories in newspapers scattered throughout Malton suggest several things that could have caused the zombie plague. Among them are a meteor shower (as per the zombie film Night of the Comet) and an outbreak of the bird flu, which at the very least could have contributed to the massive death toll that precipitated the first risings. Another theory is that the plague was caused by Necrotech, since they are the only ones capable of producing a cure.
The zombie plague is unique among known pathogens, in that its dormant period is precisely equivalent to the length of time its host spends alive. The plague is not known to have any effect on the naturally alive, as a carrier exhibits no symptoms. As far as is known, there is no immunological response to the pathogen- no residents of Malton, the plague's center, are known to have resisted the pathogen's effects. The pathogen only takes effect upon the death of the carrier, whereupon he is immediately (the time involved is so brief as to be reasonably considered instantaneous) transformed into a semi-sentient somnabulist. When the antigen is administered, however, the former victim reverts with no ill effect- but, unlike with so many other infections, there is no acquired immunity to re-contraction.
Some brief physiological theorizing suggests that the pathogen takes up residence in the brain, and is transmittable through the air- people known to have made no contact with any zombies will nonetheless be affected upon their deaths, regardless as to the cause of death. It is possible that the infectious agent is somehow stimulated by the neurological or otherwise physiological response to death- whereupon it acts. Once infected, the only known way to avoid transformation is to not die, which, in a town almost overrun by the aggressive walking dead, is significantly more difficult than most imagine.
The fact that any survivor in Malton who dies becomes a zombie suggests that the plague is very widespread. Unlike some theories about zombie plagues suggest, the zombie plague of Malton does not seem to spread solely by zombie bites. While zombies are capable of infecting survivors, causing them to die if left without medical aid, the infection does not transmit the zombie virus itself. Therefore, the infection can be considered a separate pathogen from the plague itself- most likely some sort of neurotoxin, given its strong effects and quick-acting nature.
The only known treatment against the zombie plague is Necrotech syringes. They are administered to already undead patients, and cause the patients to revert to human state. While the syringes relieve the symptoms of the zombie infection, they are not capable of completely removing the zombification-causing pathogen, and if a revived patient dies again, they will become a zombie. It is possible for the brain to be afflicted to such a great degree by the virus that it becomes next to impossible for reversion to occur. Successes have been had by high-level Necrotech operatives in overcoming the more advanced degree of affliction; unfortunately, this is only possible in a near-optimal situation.