The Virus
Assumed to have infected it's first victim in Mercy hospital in Fairfield, PA, the Green Flu is an extremelly deadly, highly contagious rabies-based virus that has developed the ability to become airborne, spreading across almost half of the eastern side of the United States in around four to five short weeks. The virus, however, is incapable of "fully" infecting specific humans, that is to say they can contract the virus by any way, shape or form and carry it, but will not succumb to the symptoms of it or become the fully-transformed 'zombies' others would soon change into. The Green Flu was attempted to be contained and/or cured by the medical research team CEDA, but the organization failed miserably and left, leaving the rest of those left behind in the chaos to die wthout a cure.
Characteristics of Infected
As it is a very close relative of the infamous Rabies Virus, the Green Flu causes its victims to become extremelly agitated, angry, and completely overtaken by adrenaline, even to the point of being able to climb flat surfaces with their hands and vault over barbed fences within seconds. Infected have also been shown to mostly idle around and wander when 'prey' is not nearby, and will even succumb to apparent stomach sickness and vomit blood. While the infected are not 'undead', they have little to no control of their brains, leading to the conclusion that they are 'living dead'. The infected seem to suffer from split-decision-making problems, as, for example, when a survivor throws a pipe bomb, complete with a flashing light and beeping sound emitter, some will run straight for the bomb, but will also look over their shoulders at the one who threw it, thus making it very possible they have uncontrollable and life-threatening Attention-Deficet-Disorder in certain scenarios, or a similar handicap lodged in thier brains as a result of the virus.
Mutations
By random chance, infected may acquire various mutations such as enhanced leg muscles for leaping, Enlarged tongues for ensnaring survivors, and the relatively rare mutations of incredible strength and resistance which makes the them extremely difficult to defeat. More mutations include acidic fluid buildup in the stomach and large, claw-like fingers.