Z-bug

The Z-Bug is the micro-organism responsible for the Zombie Plague of Y-201, the fall of some-city-yet-to-be-named, and the condition most refer to as 'zombie-ism'. The Z-bug is around 18 micrometres in size, exhibits both micro and macro behaviours, is partially conductive, emits a pheromone, is fueled by protein and fats, binds to calcium, and can go into long hibernative states.

=History=

Outbreaks
See Zombie Plague of Y-201, the Fall of Dom Ritter, and the 1st Leidian Treaty.

Identification and Isolation
The Z-Bug was first identified in laboratories in Y43, where scientists were able to isolate the strain using rats and contaminated water. Following this a number of experiments were conducted on the isolated Z-bugs, before introducing them to smaller mammals to see their effects on a macro scale.

Experimentation with the Z-Bug continues to today, but is typically heavily quarantined and monitored closely by both government and international organizations.

=Strains=

Simple Z-Bug
This is the most common Z-Bug, and exhibits all of the traits listed here and very few others. It has a very long lifespan and a long hibernation period.

=Behaviour=

Micro
On a microscopic scale, the Z-bug exhibits most of the same behaviour as a simple virus. It approaches a cell, breaches the cell membrane, and uses the material to construct more of itself. It favours protein and fat cells, however when those aren't available it will turn to other cells - notably always avoiding other Z-bug cells.

Unlike most viruses, however, it reaches an equilibrium state at around 20,000 cells per microlitre of fluid. Prior to this, it will very aggressively target cells to try and replicate; upon reaching this however it will go into dulled state. It will no longer try to replicate, but it will consume material to keep its internal processes going.

When this happens in an animal host that is producing white blood cells it is difficult for the white blood cells to actively fight off the Z-bugs due to their high concentration.

Interaction with calcium
In addition to consuming protein and fats, the Z-bug will also chemically bond with calcium, making it nearly impossible to separate. It will still consume protein and fats available to it, however it will also begin consuming the bone marrow. At this point the host's natural defenses are unable to defeat the Z-bug; too high an infection rate and the host will be overwhelmed.

Macro
Macro behaviours are exhibited by a Z-bug collective, which is a group of Z-bugs which have reached equilibrium. When this happens inside of a human body, it is the first step towards full zombie-ism, and is the first point at which the host begins to lose control. Macro behaviours are caused by the Z-bug collective lurching towards the same goal, communicating through electrical signals passed from individual micro-organism to the next.

The stages listed here are also the three stages of zombification.

Stage 1: Spasms
The most basic electrical signals cause twitching and spasms in flesh. At this point, the Z-bug collective's edges (which are not in equilibrium) will continue to seek out proteins and fats to replicate. Doing so they will pull the entire collective along, communicating the macro movement backwards. This causes the basic 'lurch' movement, which is why most fresh zombies are seen shuffling or crawling.

Stage 2: Coherent Movement
As the collective grows larger micro-organisms in the equilibrium will become more specialized - building off the micro-organisms that are bound to the bone, they will become elongated linkages to mimic muscles. At this point movement of the host becomes more evolved - a shambling gait or reaching and grabbing of the hands.

Still driven by the need for flesh, the Z-bug collective is mostly entombed within the dead host. They cannot simply 'absorb' another piece of flesh, and will instead try to break into it - through biting, grabbing, or beating. Once they've broken through the skin layer Z-bugs will infect the victim's system, and the Z-bug is passed on.

Victims that don't survive an attack don't fight of the Z-bugs, which accelerates the zombification process. Victims that do survive are often too infected to fight off, and will frequently succumb later.

Stage 3: Hardening
The final stage occurs after the last of the host's flesh has either been consumed or decayed. In this stage, all that is left is the Z-bug and the host's skeletal structure, which it has bound to and continue to uses. A Z-bug collective without a flesh encasing typically dehydrates from the edges inwards, forming a thick hardened shell that appears pale blue.

Inside the hardened shell the Z-bug collective still responds to external stimuli, though the dehydration hinders movement. At this point the zombie will begin to decay entirely, losing movement capability before ultimately collapsing. However, when it collapses it will begin to seep into the ground. Once in the ground, the Z-bugs will slowly seep into the water table, where they can begin infection again once consumed by another animal.

Hardened Zombies are thus one of the most dangerous biohazards to clean up, as they are essentially a walking bag of infectious material.

Stimuli
Z-bugs respond to changes in light, sound, and smell through a series of very primitive sensory organs. When these stimuli occur, the Z-bug will communicate it to surrounding Z-bugs through electrical discharge. These stimuli will typically attract the attention of zombies because of this.

=Other Animals=

Z-bug is particularly effective large, slow moving animals like elephants, but completely ineffectual against birds. While an animal might be consumed it's mobility is still limited by the bone structure.