Screw your weird names for zombie viruses. I don’t like them and think it’s strange enough that people can quantify such freakish effects. By the same token, forget that it’s impossible to trace the beginning. Infection can be traced to an artificial chemical that’s used in some common food products—but not with a 100% infection rate, thankfully. Bites are not 100% infective, either, and the virus can be immunized against, vaccinated, et cetera.
At the moment there is no vaccination for it in medicine, but a person consistently exposed has a chance of surviving with other medications and an incredibly strong immune system. They will likely be subject to a very rough struggle lasting days, if not weeks, and they may just be eaten alive during this time. A person totally immune would be very rare, if not entirely theoretical.
The reason that the virus isn’t 100% effective every time is because many have been exposed to it before, and only recently have they really been dropping. In the past strange happenings have been covered up to prevent mass panic, but it’s now knocking at the doors of the populace.
The walking dead are not supernatural. They do not last for five years. They last until they decompose—which isn’t that long, honestly. Their endurance is not inhuman, it only hurts them in the end when they become too weak to move. They are susceptible to cold and rigor mortis. Essentially, they are no better than your average cadaver.
Undead may retain some recollection of their living years. They may not hold true societies, but are drawn to one another—interesting phenomena like families grouped together or partial communication may be seen. They are not entirely single-minded, though the living take priority above all else. Even then, it seems the corpses are difficult to think of as human.
It may be possible to reason with a body, though such situations are far and few between—for example, a zombie locked up in chains and forced to see objects from its living memories every single day may eventually make a connection between itself and those objects. But that would be something purely for sport.
The undead themselves, if you happen to find yourself playing one, suffer from an intense hunger that cannot be sated except during the actual act of eating. They have some instinctual basis of thought patterns and are more likely to think via pictures than words. Very little concept of language left. They do not retain much of their conscious memory, but it may be brought back—and they do have some manner of emotion. Undead can feel loss and emotional pain, though not physical versions thereof.