The Harran Virus is a rabies-like pathogen that has spread to epidemic proportions within the city of Harran.
The virus is mentioned in-game as being an apparent mutation of rabies. The virus is light-intolerant, being killed when exposed to ultraviolet radiation, and, consequently, sunlight. This light intolerance is also exhibited by some of the more mutated victims of the virus, with some of these creatures being nocturnal and being repelled by the UV Flashlight, UV Flares, and the UV lamps that surround Safe Zones.
The bite of an infected subject, being direct fluid exchange, is fast enough to guarantee infection before UV radiation can kill the virus. However, the spread of the virus within a victim's body can be stopped if the bitten limb is amputated quickly enough, as was the case when Karim had Erol Asani's lower left leg amputated shortly after he was bitten, thus saving Asani from infection.
The virus seems to cause a hyperactive reaction of the fight-or-flight response in victims once they pass from the initial symptoms to full-on zombified state. In most cases, victims exhibit hyperaggressive behavior, but certain types of infected (bolters, screamers) seem to have a heightened flight response.
The virus is also not like your average zombie infection. Similarly to The Last of Us's Cordycep's Brain Infection, infected hosts still somewhat rely on other parts of their body than their brain to survive.
Transmission occurs via the introduction of an infected individual's bodily fluids into a healthy person's bloodstream. The virus is likely replicated in the infected's saliva and/or blood. Due to the primitive (and highly aggressive) behavior caused by the Harran Virus in infected individuals, the typical method of transmission is through biting.
Before being completely overtaken by the virus, infected will periodically suffer from seizures causing fatigue and blurred vision. In late stages the individual will have a pale complexion and sometimes appear malnourished. The time from infection to transformation seems to vary greatly from hours to days., though the transformation time could depend on other factors, since Rahim turned into a zombie in less than an hour.
It it not explicitly stated nor shown in Dying Light what happens to an individual throughout the process of turning, however long it may be. After turning, it is shown throughout the game that the first phase of the virus leaves the host with most motor skills intact, coupled with an uncontrollable rage and hunger for flesh (in keeping with Zere's finding that the virus is a mutation of the rabies virus). It is implied, due to the rate of behavioral degeneration between virals and biters, that the virus attacks the neurological system, particularly the brain itself. Biters display no ability to sprint or run at a controlled cadence, suggesting that the virus is degenerative and breaks down brain matter throughout the process of infection. It is also not stated how an infected person becomes a volatile, a toad, or a bomber, as opposed to eventually deteriorating into a biter. This suggests that the Harran virus itself carries more than one mutation, and varies from person to person. But it is possible for this virus to be cured from a Poloviraus. This will cause the zombie or Volatile or any other substane of the virus can be cured, It will cause them to morph back to a human version of themselves.
The Harran virus, despite how lethal and unpredictable it is during the outbreak, does have one weakness - Antizin. Antizin is a viral suppressant which temporarily stops the Harran Virus' progression and relieves symptoms, it is however not a cure.
When infected, an individual will start having seizures. If Antizin is not administered quickly, the individual will begin to transform into an extremely aggressive "Viral" with 30% exhibiting human speech for a short period before returning to their natural state of aggression. After a period of time, they will deteriorate into biters. Biters are mindless animals, attacking, and biting, anyone not already turned.
According to David, a French conspiracy theorist, the epidemic occurred due to an alien race known as the Gadoids, who introduced the virus through a meteor which impacted Harran. If this is true, then the virus came from extraterrestrial origin, this isn't an impossible theory, as a crashed UFO can be found underwater. However in The Following it is presumed the military are responsible for the outbreak, It is shown during the mother's flashback when she drank a tube containing a blue liquid it changed her into a sentient volatile.
Biters are mindless zombies, and roam around Harran searching for flesh to eat. They are, like all zombies, sensitive to sound, and will follow a sound to its source. They wander aimlessly and sluggishly, incapable of moving faster than a sluggish walk until they reach their target and lunge at them. Biters appear to be affected by sunlight. During the day, they remain slow and predictable, but during the night, they display greater aggression and more agile movement. They are easy to kill, with most being brought down simply with a blow, stab, or shot to the head. Larger biters are more resilient to combat, and will require more damage to bring down.
It should be noted that, in The Following, some biters will begin to convulse and vomit uncontrollably, signaling that they are about to display viral capabilities. However, they are easy to kill in the process of this transformation, assuming they do not turn before they are attacked - This is a hint that whilst degrading due to the main strain of the virus, there is an intermittent phase where an infected can change back to their Viral phase.
Virals are infected and have recently turned. While they do not display restraint towards assaulting their victims, they have been shown to plead for mercy when being attacked, shortly before resuming their barrage. Virals are incredibly aggressive and have the ability to sprint after their targets and give chase. Virals will not latch onto or bite their victims as biters do; they will attack with their fists until their target is dead. Virals are highly sensitive to loud noises, and this will attract them to the location of the sound. Virals are capable of dodging players attacks, which makes them more a bit more difficult to kill. The nature of virals is not unlike volatiles, hiding in dark places or Volatile Hives. Virals in theory if not killed will eventually degrade into their Biter counterparts due to the base virular strain causing decomposition to take effect on their bodies.
Toads are capable of attacking at a range, preferring to spit toxic bodily fluids at a distance to direct combat, hinting that their mental faculties are slightly retained in the means that they do not like engaging in melee combat. The toad's toxic attack is capable of taking off health over a period of time. While they prefer to attack at a distance, Toads are very easy to kill. A few well place attacks with strong enough weapons will kill them quickly. Toads are rather weak and are easy to dispatch by the means of more powerful melee attacks from two-handed weapons such as hammers or large swords or thrown weapons like stars or axes.
It is not known what it is that makes bombers explode (scientifically) however they do explode due what is seemingly their exposed and rather fragile bulbous-stomach lining which effectively acts as a namesake bomb - It is possible that their mutation of the Virus is brought on by an immense swelling of the stomach and intestinal area (almost like a very radical and unrealistic case of gastritis). They run at a brisk jog at their targets, begin to convulse, and explode. The explosion triggered by a bomber is devastating, destroying everything within a three-meter radius, according to Karim. Pretty much any offensive-means of attack will cause bombers to explode i.e. thrown weapons, gunshots, arrows, crossbow bolts, being hit by vehicles (such as the Buggy) or even being punched by unarmed survivors.
Similar to the Toad and Bomber, the means of infection from the Harran Virus is largely down to the specific mutation that transforms a human into a Goon (Goons are roughly between 7'0" and 7'6" tall so this mutation is likely to drive the victim's growth hormones into overdrive). Goons are much bigger than Biters and most other infected variants and is very sluggish in movement, being slower than the Biter. Goons however prove to be more powerful as they are often equipped with a two-handed weapon (which shows possible signs of their mental faculties having advancement in respect to them identifying what to use as weapons). Whilst being slow, they do require caution on behalf of survivors as Goons can cause immense damage per hit they land. It is also possible that Goons are similar to the larger Biters albeit haven't degraded due to their mutation of the Harran Virus allowing them to retain some mental faculties.
Far bigger than a Goon and the largest variant of infected, this mutation of the Virus is most likely brought on by a similar manner which explains the Goon variant's of the virus. Demolishers however have the ability to move quickly through their charge attacks whilst Goons slowly move and drag their weapons along the floor. Demolishers also seem to have more aggression and mutations focused on strength and muscle mass given that they can rip out concrete slabs from the ground and hurl them at survivors. Demolishers are at least 10 to 11 feet tall (regular Demolisher variants) and they are always seen in some form of security uniform and armor, hinting that their original human form prior to infection was that of a security guard/law enforcement officer.
Retaining mental faculties to their human form, Screamers are children that have been infected and are "zombified" as such but they do not attack in other methods exhibited by other infected variants. Instead they use their namesake attack to stun and cause shockwaves of damage to non-infected survivors by screaming at incredibly high pitches - Otherwise when unaware of survivors, they can often be heard crying and sounding a lot like a non-infected child does when crying. The Screamer's scream/stun attack is so loud that it will attract many Virals to the area where the Screamer is, often making Screamers a technically dangerous type of infected to deal with if the Screamer is startled by survivors. Screamers are not confrontational or aggressive, but seem to be defensive as they do not go out of their dwellings to join the rest of the infected populace - Yet again more signs of mental faculties retained and perhaps retained memories of where they lived or were prior to infection.
This type of infected shows 100% "flight" characteristics in the way that it is never aggressive and is very cowardly to pretty much most if not all things, including survivors. Apart from in the Countryside, Bolters only come out at night and are terrified of survivors/non-infected beings. Mental faculties are retained to an extent in using other stronger infected for protection i.e. Volatiles, that will be seen wandering around the feeding grounds of Bolters marked around Old Town and the Slums of Harran. Bolters must have a similar strain of the Virus to how Virals have, albeit without the full matchings (or else Bolters would degrade and transform into regular Biters) as Bolters can move extremely quickly when fleeing survivors who will often hunt Bolters for their valuable tissue and liver. Bolters can move extremely fast and even a Level 2 overall Buggy will struggle to keep up with a Bolter unless at speed and on a straight pathway/road.
Easily one of the most dangerous and disturbing variants of those infected by the Harran Virus, a Volatile has extreme strength, speed and agility plus the ability to use acidic phlegm (similar to a Toad) as well as pounce-attack survivors. Volatiles come in two variants - One is the regular and the other is the Evolved/Alpha Volatile albeit the Evolved/Alpha variant is simply bulkier, has more resistance to damage and can see/detect if survivors are trying to camoflauge themselves/blend in with the infected. Volatiles are seen to retain mental faculties in the means of knowing what harms them the most, that being ultraviolet/UV light/UV radiation and sunlight, thus why Volatiles only come out at night or are in dark areas during the daytime. Volatiles also have a pack-mentality (as seen during the main story quest "Broadcast"). Plus the Volatiles only seem to be generated by converting from a human-form to their own unique humanoid mutation of the Harran Virus (such as the case was for Sabit when he was seen developing/being changed/converted from a human into a Volatile). Therefore, it is very likely that the Volatiles only become how they are by being initially infected as humans and prior to fully transforming into an infected, they are put into a Hive and develop (yet again just like Sabit was).
A type of infected which is potentially one of the most lethal and dangerous and aggressive (depending on who has control of the Hunter), the Night Hunter is a unique type of infected as it can use tendrils, sprinting, parkour and swimming to move whereas the "one-down" Volatile infected variant can only sprint and parkour (this is because a Volatile doesn't have tendrils for traversing the environment nor can they swim - In this respect the Night Hunter is the only infected variant that can swim). Night Hunters have the prime instinct of hunting survivors (namesakingly) and protecting other infected (mainly incubating/developing Volatiles/Volatile hives) and only exhibit "flight" behavior when exposed to UV light as exposure to UV light drains their energy and prevents them from fast movement as well as renders most of their attacks/means of defending the developing Volatiles useless. It is not known what caused the Night Hunter strain of the Virus albeit some speculate it to be Kyle Crane after the "Infected Ending" of The Following.