This article uses an in-universe perspective.
Solanum is a common zombie virus in The Zombie Survival Guide.
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, individuality, mercy and according to many, humanity, is irrevocably lost. The viral incubation period is variable, with some turning rapidly (most common in bite victims) and others not showing symptoms for a significant period. Combined with the ability to spread via non-obvious means, such as open cuts and not just clearly visible bites, this necessitates the careful screening of individuals before allowing them into safe area.
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 virions and infected 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)
(Take, for example, The Zombie Survival Guide by Max Brooks. The first few chapters talk about the causes and symptoms in depth).
Point of OriginEdit
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.
However, documentation published in The Zombie Survival Guide point to experiments conducted with Solanum conducted by China after World War 2, using research taken from Japan.
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. Because of their strong connection to humans, dogs are shown to be the sole non-human animal that will stand their ground against zombies, allowing them to receive specialized training to become scouts for infected areas. However, even this exception is rare, with most dogs failing the selection screening for scout training.
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 infection 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. Testing has consistently shown 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 vector 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 infected, little can be done to save the victim. Scientific research has provided little evidence that a cure exists. Immunization is useless, as even a single virion will lead to a complete infection, as the virus can apparently mutate to resist the human immune system.
Genetic research intended to form stronger human antibodies, develop virus-resistant cell structures, or counter-viruses has been unsuccessful. Removal of infected limbs or other appendages immediately after a bite has saved individuals on rare occasions, but given the speed of the circulatory system, successful amputations are uncommon. More often than not, the victim is inevitably doomed after being bitten, regardless of bite location.
Should an infected human choose suicide, they must destroy the brain to prevent reanimation; even if the circulatory system fails, the virus may still move to the brain and reanimate the body (a bit more slowly). Anyone killed after being bitten or otherwise infected by the undead should be immediately disposed of and destroyed completely, preferably by burning.
Infected individuals can continue to mingle among uninfected humans for some time, depending on the rapidity with which the virus is able to reach the brain. Screening is thus necessary to ensure that persons exposed are not in the early stages of the virus, which are symptomless. It is also necessary to distinguish Solanum infections from other diseases, as pre-death symptoms can be similar.
In the case of transmission via bite, detection is relatively simple. That being said, due to human nature's self-preservation instincts, many victims will try to hide their bites, thus making voluntary disclosure unreliable. In the case of alternate means of infection, such as contact with infected bodily fluids, the person may be unaware of the danger they pose.
The best detection method thus far found is the use of trained dogs. Dogs will bark or, if sufficiently trained, act extremely warily around infected. Dogs born pre-outbreak would uncontrollably attack any infected regardless of training, turning on handlers if they attempted restraint, and thus when used as detectors they were kept in cages. As the amount of ambient material from infected (via decomposition, burning, etc.) increased, many of these animals were driven into permanent attack rages, and had to be put down. Post-outbreak dogs are more used to the virus due to it's omnipresence from birth, but a majority still have trouble controlling themselves around active zombies. Those who do not are recruited for the K9 corps, while the rest are used as guard dogs.
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 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.