Zombie Science
I got ridiculously excited about this one, which I wrote out as a counterpoint to the various theories that green flu was created as a bioweapon. Road to hell, good intentions and wot.
THEORY: Green Flu is a viral vector and the pandemic spread from a clinical trial that used it for gene therapy.
Gene therapy is meant to replace or repair a missing or mutated gene or to make diseased cells more obvious to the immune system if it does not recognize them as damaged. A new gene can't just be inserted into a cell though, it is spread through the affected cells using a vector, which is something that can carry something into a cell. Sometimes the vector is stem cells, or liposomes (fatty particles) but most commonly the vector is a virus.
A disease causing virus reproduces by injecting its own genetic material into a living cell, which will later rupture and die, spreading more virus. A vector virus has been altered to inject the new repair gene instead of its own material. It changes the cell the way a regular virus does, but instead of killing the cell it makes it more effective.
This has been very effective, but there are three major concerns here:
Immune Response: your body might react to the viral vector the same way it would to a normal virus. You can't just explain to a person's immune system that viruses are bad but THIS virus is okay. This can cause the treatment to become toxic and in severe cases can cause organ failure as the patient's body rails against a threat that never meant it any harm.
Viral Spread: Viruses aren't always picky about what type of cells they infect, and a viral vector might end up infecting a bunch of cells that it wasn't mean to. If it starts trying to "repair" cells that were already healthy and damaging them, it can cause more problems than it started out to fix. Complications can include cancers forming as the newly damaged cells multiply. This is what we might be looking at with the zombies.
Reversion: It is possible that once the vector is introduced to the body, it may regain its original ability to cause disease. This is the other thing we might be looking at.
So picture this:
A clinical trial is introduced for a new gene therapy. It looks very promising and a large sample of people with the given disorder is used. Control and treatment groups are established and the trial begins.
At first all seems to be going well, and the treatment group shows a marked improvement over the control.
A number of patients in the treatment group begin to report side effects, mostly flu like symptoms, and an occasional increase in aggression, and there are some concerns that the vector has reverted. It appears to be stable, however, and in any case it seems to be nothing much serious.
Hell happens here: the entire treatment group starts getting horribly sick, and even more alarming, their families and friends are reporting symptoms now too.
The viral vector has mutated and begun spreading indiscriminately. It enters a period of wild mutation and rapid reproduction, evolving over only hours or days. It first becomes airborne during this period, and continues to spread very efficiently from all exposed individuals. There may be asymptomatic carriers, but there is no immunity: this is a novel virus, step one to a pandemic.
Five people in the treatment group turn up with very different symptoms than the others.
Four male, one female, and not one of them has the same thing. They seem to have nothing else in common, either, except that they had the problem and had the treatment.
These are the original special infected, and in each of them the rapid evolution of the original vector is affected strangely by some factor unique to them. The collective disease known as green flu is now actually comprised of no fewer than six different related viruses, one common and widespread, originating from the bulk of the treatment group, and five rare and originating with five individuals.
New mutations begin to die off and the virus slows its evolution, though several of the more unstable strains continue to change as they spread: the Charger and Jockey strains are offshoots of the Tank and the Spitter is a mutated, imperfect version of the Boomer. The virus at large enters a "stable" period. It now has at least eight widespread strains and many more isolated types that didn't spread as efficiently. (Regional or "uncommon" strains)
Attempts are made by CEDA to circulate a vaccine, but the disease is simply too unstable for this to be effective: it may protect against the original common strain but has less or no effect on anything else.
Also, despite previous speculation on my part that it started out as rabies, Green Flu may actually BE a flu. A normal flu already evolves very quickly, and a highly altered, partially reverted and much mutated flu might be able to produce the HYPERSPEED evolution we see with this virus.
(influezna viridis? That is NOT how viruses are named but it sounds cool)