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, light traps 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.
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.
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 agressive "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.
Biters are infected that have been turned into mindless creatures interested only in feeding off the flesh of those not turned. Biters also seem to have very acute hearing, as any loud noise will draw large hordes.
The virus appears to cause a wide variety of mutations in infected individuals. It is important to note that simply being infected does not prevent attacks from biters - though daring individuals may cover themselves in the guts of deceased infected to temporarily move undetected. They can be killed quite easily.