Referred to as the the Great Yew Plague, the Living Death, or just “The Plague,” the disease that has decimated the population of Sosaria is known to have first surfaced roughly 80-90 years ago in the north-eastern regions of the Protectorate of Yew. Because the villages and towns initially affected were quickly overrun and destroyed by the resultant hordes of undead, the true nature and danger of the Plague was not immediately apparent. It was only when the growing horde began to eat its way into the more populous areas of Yewish and Covian territory that it was finally linked with the surge in undead activity. Though the military response was quick and initially quite effective, the sheer, daily-increasing number of plague zombies gradually overwhelmed the troops assigned to contain it.
Unable to prevent the disease from spreading into their populations, the major nations - Cove, Yew, Trinsic, and Vesper - were only just barely able to prevent the plague hordes from overrunning their central provinces. Only a bitter, costly defense and an early winter freeze prevented the fall of Cove and Yew, while Trinsic's tropical climate afforded it little respite and it was almost completely consumed. Vesper's location relatively far from the epicenter of the outbreak protected it from the brunt of the first outbreak's effects, leading to great suspicion that their heavy use of dark and dangerous magic had somehow played a role in causing the plague.
Over the eighty years since the First Plague, there have been no fewer than ten further outbreaks of varying severity and scope, and the population is now a fraction of what it was before the outbreak. Several cities are known to have been completely abandoned or consumed, while the fate of others - such as Yew itself - are unknown, and Cove is known to have managed to weather the storm despite horrendous and crippling losses. Regardless, those few settlements that have survived are now hopelessly separated from one another by a sea of enemies that've occupied the ruins of human civilization, and those few of the Great Cities to survive are scarcely capable of defending themselves, let alone their neighboring towns and villages.
These are the stages that manifested in the plague as it was observed during the first outbreak. The proceeding outbreaks have had a large degree of variance, and the timetables for each strain can vary wildly. The symptoms and results of each stage are, however, generally constant (if not even more severe).
The subject begins to develop a fever and a dramatically increased appetite a matter of hours after initial exposure as the infection takes root in their body. Prior to this, the disease may be prevented from taking hold by thoroughly disinfecting any and all wounds. Towards the end of the second day, the subject begins to develop severe abdominal pains, signalling the start of several fatal mutations. At this point it is still possible to cure the patient, but only by administering an extremely difficult to produce medicinal potion which nevertheless only has a small margin of success (20%) and requires that the patient be allowed to enter the contagious phases of the disease before its success or failure is known.
The subject begins to lose control of his body as the disease alters it for its own purposes. The heart and stomach become bloated and mutate to increase their power and capacity, while other organs shrivel and atrophy to make room. The brain undergoes a series of changes that override all impulses other than the desire to feed and a compulsion to spread plaguebearing material - blood, mucus, pus, bile, saliva - to uninfected potential hosts, completing these changes and completely burying the will of the victim by the end of the fourth day. By the start of the third day, the disease is beyond the curable stage.
By this point, the victim is little more than an unnaturally bloated and mutated husk. Definitions of life and death become muddied at this stage, as the human qualities of the victim are all but extinct, having been replaced or repurposed to facilitate the propagation of the disease, but the body is still animated and capable of rudimentary (if narrow-focused) thought. By the fifth day, the victim's hunger has become one for flesh, and their entire will has been diverted to the pursuit of that goal. From this stage onwards, the withering of various secondary organs has caused the body to begin to rot, and the state of decay only worsens as time passes. Eventually, the corpse rots to the point of being unable to carry on its shambling quest for flesh, but remains capable of transmitting the disease until all bodily fluids have dissipated.
The Plague spreads exclusively through fluid contact, whether it be blood, saliva, bile, pus, or mucus. Any such contact has a chance, proportional to the degree of exposure, to transmit the disease. Ingesting plaguebearing material has a 100% infection rate unless vomiting is immediately induced - even then, enough of the material may remain in the body to create the risk of infection.
(Hard Mode: Roll a saving throw on a d100, needing X+1 to prevent infection, where X is what you estimate to be the chance of infection considering degree of exposure. For instance, getting a bit of blood on a recent, still-open scratch or scrape might result in a 30%, requiring a 31 or better to prevent infection.)