The mystery of the Harran Virus is very intriguing and the story drops numerous clues about how it all went down but never gives us a definite answer.
Lets start from the main campaign. The GRE wants us to retrieve a file which contains the "cure", but as we all know, they want to weaponize it ! Now, the GRE logo says, "since 2002", which means it was probably not always the "bad guy". Dr Zere explains us, that the virus is rabies like and we know that when a person is infected, he/she turns into a Viral, eventually decaying into a slow Biter. So where did the Volatiles come from ? Surely they didn't just pop out of nowhere right ? In the beginning of the outbreak, the GRE sent Amir to assist Dr.Camden and Zere while they were figuring out a cure, so they definitely were interested in curing the virus at first.
My guess is that, the GRE set up a research facility in the countryside(Open fields, lower population) to work on a cure. Now, while studying the virus, they realized that it had the ability to mutate and give the host: superior strength(Goon and Demolisher),night vision(All Zombies) and better agility.
This "breakthrough" led them to think that they could create a serum (Similar to the super soldier serum in captain america) So they reverse engineered the virus and tested it on a few human test subjects.The result was a volatile ! It gained all the abilities they had hoped of but the virus took over the host... making it an unsuccessful and dangerous product which is why they had nuclear warheads as a backup plan to destroy all evidence and traces
They still carried forward the research and it led to another breakthrough ! The test subject had a human mind when exposed to UV Light, but in the darkness, the virus took over again.(We are looking at the sentient Volatile)
At this point, there was an outbreak in the lab that couldn't be contained and the volatile test subjects escaped and started breeding in hives (Remember, the body still functions which explains why they don't decay into biters) This is how the volatiles came into existence. (A natural virus was reverse engineered in the wrong way)
The countryside is littered with hives because it was where the volatiles first appeared ! However, the Harran Virus did start in Old Town/Sector 0.
The "mother" in the following was the wife of a colonel which explains how she got access to the serum.Thus, when the GRE said that if the "cure" in the stolen files was used at its current state, it would have bad consequences, they were right. Indeed, the file that Rais stole, had the recipe for the sentient volatile serum
This is why, Volatiles and Biters are from the same base virus, but entirely different.This also explains why volatiles only kill, and not bite.The scientists probably managed to change some traits of the virus. The Biters are undead, while the Volatiles are living (But not Human) and thus can also breed. Also explains how the average human became such a monstrosity.They are a completely different species !
Maybe, Dying light 2 could have us fighting a super soldier Zombie from the GRE (Boss Fight) !!