On the Neurology of the "Undead"
The following document was found in a composition notebook in the ruins of the Hurst Building. Although there was no name accredited to the document, the handwriting and word choice strongly imply that it was the work of Dr. Ashar Alhazrad.
The interesting thing about zombies is that the shift in behavior following the transformation into one of the undead is the easiest thing to comprehend about them. In fact, it can all be ascribed to a few simple changes to the endocrine and nervous systems.
The first thing that occurs upon "zombification" is a breakdown of the epinephrine-norepinephrine feedback cycle. Epinephrine, more commonly known as adrenaline, is a hormone that increases the rate of cellular metabolism, providing a burst of energy to the the body. In high levels it can also have strong neurological effects: anger, extreme hunger, and a lack of response to pain. Normally the body has a feedback system to prevent such high concentrations from remaining in the system long. The hormone norepinephrine, which has reverse effects, is produced when the body detects a high level of epinephrine. But for some reason, this doesn't happen in the body of the zombie. I don't know if this is due to an inability to produce norepinephrine or a loss of the ability to recognize high levels of epinephrine. The blood work I've run can't tell me that. And I'm a bit afraid to try an autopsy since that last one regenerated on the slab.
Another effect that I have observed is that seratonin levels in the brain drop dramatically. In clinical studies it has been found that low levels of seratonin correlate strongly to sociopathic behavior. I'm a molecular biologist, not a neurologist, so I'm not sure why that is. I'm just going on what I've read. But it still is important to notice.
Finally, when I managed to wrestle a zombie into the PET scan machine, I noticed that activity in the frontal lobes is remarkably lower than what would be seen in an ordinary human. The frontal lobes are the seat of the majority of abstract reasoning. Cases of brain damage that affected the frontal lobes have resulted in people incapable of taking any course of action other than the most obvious. Give them a lighter and a cigarette and they'll smoke, even if they normally wouldn't. That's just what you're supposed to do in that situation.
So take these three factors together and what do you get? A very angry, hungry person who is incapable of abstract thought and doesn't see why it is wrong to eat people. They aren't exactly savage, it's just that they will always do the most immediately obvious thing to satisfy their hunger and rage. And that's a zombie. There may be other factors involved, but I believe that the symptoms of the L-strain that I have observed can explain most of zombie behavior. The one thing I don't understand is why Agent 51 negates the neurological effects of the virus. Really, I don't understand anything about how Agent 51 works. Clearly I'm going to have to do some work on it. Maybe I can mutate Agent 51 a bit, examine how it works by trying out different strains. But, then again, NecroTech retroviruses have never gone well with forced mutation.