The initial clinical trials of the Gamma Strain vaccine were successful, however before the vaccine went into full production, it was tampered with. This manipulation had a devastating effect, and is now referred to as the Gamma Strain Mutation.
The effects of the Gamma Strain Mutation can vary from case to case. Subjects are stimulus-response creatures that seek flesh, implying that the pathogen has acted to re-animate the existing neural pathways and motor functions in some way. The sensory systems that are re-activated are capable of distinguishing four key elements: flesh vs non-flesh; species, infected vs uninfected. The virus does not create any new systems, but merely hijacks existing ones.
GSM begins to transform the host from human into a zombie once it is introduced into the body. Through means not yet fully understood, the virus uses the cells of the brains’ frontal lobe for replication, destroying them in the process.
All case studies have shown that contracting the Gamma Strain Mutation (GSM) is terminal. It is presumed, in theory, that the hijacking process can be slowed or even halted with proper medical treatment, although no case studies have been linked to verifiable proof. Vyal Pharmaceuticals is currently working on an antidote for the GSM in their research facility in Upstate New York.
It is assumed that the infection of the medulla oblongata, where most automatic processes are regulated, is what results in cardiopulmonary death, followed shortly by brain-death. Reports vary on re-animation time, from between 5 minutes up to an hour, after brain-death, the medulla is then fully hijacked by the pathogen.
Infection can occur only through direct contact with bodily fluids. A zombie bite, although by far the most recognizable means of transference, is by no means the only one. Humans have been infected by being scratched by a zombie, brushing their open wounds against those of a zombie or by being splattered by its remains after an explosion. Ingestion of infected flesh (provided the person has no open mouth sores) does not cause infection, but rather, permanent death. Infected flesh is found to be highly toxic.