This article uses an in-universe perspective.
Solanum is a common zombie virus.
Solanum 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. Once the brain is infected, all bodily functions (including function of the heart and lungs) cease, and all trace of personality, individualism, mercy and according to many, humanity, is irrevocably lost.
The transformed brain does continue some minor activity, as the zombie will reanimate and hunt and consume anything it identifies as living. Its' preferred prey is humans, choosing to chase a fast human over any animal that it might catch more easily. Some bodily functions remain constant, others operate in a modified capacity, and the remainder shut down completely, which is why zombies are commonly referred to as the living dead.
The virus mutates each infected cell into a sort of organ, independent from every other cell, and the physiological tasks it once performed for the human. They also produce a great deal of oxygen, which circulates throughout the body. By removing the need for oxygen, the undead brain can utilize, but is in no way dependent upon, the complex support mechanism of the human body. The energy source of the zombie remains a baffling mystery to science, as a zombie will continue moving indefinitely without food, water, or rest. Even when a zombie does "consume" an organism, solanum cells do not draw energy from the flesh like living cells would. The flesh merely remains in the digestive tract of the zombie until it rots, or newer flesh presses the human remains out of the anus (or if the zombie has torso wounds, a rupture in the abdomen)
Despite exhaustive (and continuing) searching across the world, no isolated sample has ever been found in nature. The source, and most of the abilities of the virus remain unexplained.
Post World War Z research suggests that the pandemic that killed 50-70% of the world's population originated from the Yang-tze River in China, somewhere in the flood basin of the Three Gorges Dam. The exact source is still unknown.
Solanum is 100 percent communicable and 100 percent fatal. It is not waterborne or airborne, and perishes rather quickly in an open environment. 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.
All animals besides humans can instinctively detect traces of the Solanum virus, and nearly all have the same reaction to it: terror. From ants to whales, nearly all living creatures will flee in terror from the zombie virus. Solanum is fatal to all living creatures, regardless of size, species, or ecosystem. Reanimation, however, takes place only in humans. Studies have shown that Solanum infecting a non-human brain will die within hours of the death of its host, making the carcass safe to handle. Infected animals expire before the virus can replicate throughout their bodies. Infection from insect bites such as from mosquitoes can also be discounted. Experiments have proven that all parasitic insects can sense and will reject an infected host 100 percent of the time. It would seem that Solanum is custom made for the human race, since reanimation only happens in humans, human zombies are the only medium through which the virus can spread to new hosts. This could explain why zombies seek to eat living animals, with a preference towards human flesh over other animals.
Once a human is infected, little can be done to save the victim. No progress has been made on a cure. Immunization is equally useless, as even a single virus will lead to a full-blown infection.
Genetic research intended to form stronger human antibodies, develop virus-resistant cell structures, or counter-viruses have all been fruitless. Immediate removal of the limb bearing a zombie wound, keeping the virus from reaching the brain is an option, but due to the willpower to do this, and the speed at which the heart pumps the virus through the circulatory system, successful occurences are uncommon. More often than not, the victim is doomed after being bitten, regardless of bite location.
Should the infected human choose suicide, they must destroy his brain to prevent reanimation, as although the virus uses the pumping of the blood to speed up locomotion, it can spread through its own means (albeit more slowly). Anyone killed after being bitten or otherwise infected by the undead should be immediately disposed of and their brain matter destroyed.
Solanum and DecompositionEdit
The toxicity level of undead flesh, combined with the 100% fatality rate for infected lifeforms significantly slows down the process of decomposition. This is because all organisms, even the bacteria that play a role in natural decomposition, are also killed when in contact with Solanum. This has the noteworthy side-effect of making many zombies practically odorless, due to the absence of the bacteria responsible for body odor and the traditional smell of rotting flesh. Zombies that have been undead for many months may start to show minor signs of topical decay, and the recently turned may smell because humans void their bowels after death. Other than that, zombies produce no odor.
Solanum and the Already DeceasedEdit
Zombies ignore dead flesh. While they do continue to consume their prey until it is horribly mutilated, they would pass a fairly fresh corpse that it had not been hunting, even if the body is preserved (such as a heart attack victim). Likewise, experiments where Solanum is introduced directly into the frontal lobe of a corpse dead only minutes show that Solanum only reacts with living cells. Therefore Solanum does not create "life"- it alters it, and any attempt to use it to reanimate the dead is futile.