Vampire Virus

The Vampire Virus, also known as the Vampire Plague or Vampirism, is a highly evolved virus that turns its hosts/victims into Vampires.

Spread through the ingestion, injection or transfusion of blood or saliva of a vampiric individual, the virus is said to have an incubation time of five days but the physical transformation that the virus renders upon the host can take anywhere from a week to a month to complete. During this time, the infected will begin to feel dazed and faint, become sensitive to ultraviolet light, and exhibit mild flu-like symptoms as the virus alters their DNA.

Despite claims to the contrary in old folk tales, the vampire virus is fairly weak and only able to infect the body of a host that is already near death as the immune system of a body at full health will easily dispose of the viral infection. To this end, if a vampire desires to infect an individual they first feed on them until their victim has lost enough blood to be near death. With their white blood cell count low and their body otherwise preoccupied with trying to sustain itself with depleted resources, the virus can freely infect the individual with virtually no resistance from the individual's shattered immune system. When the conversion process is complete, the victim will become a classification of "Vampire".

Trivia

 * While human mythology only covers human-turned vampires, Wladislaus Drakulya implies that other species (such as Harpies) can become vampiric beings just as easily as humans.
 * The Vampire Virus also gives the host's a slight increases in levels of strength, speed and endurance, but in doing so, it also speeds up its hosts' metabolisms, to the point that a host will starve to death if they spend three days without consuming any blood. But the increase in metabolism also means the host's regeneration factor is increased; enabling them to sustain damage that would normally be fatal to a non-vampiric being of their species.
 * Although the Vampire Virus turns a victim into a vampire and thus a carrier of the virus, Curie´s Secrets reveals that the victims will not actually sprout wings or fangs. If you proceed from this logically, modern vampires like Curie and Wladislaus are probably only bat-like extra species which were infected by a now extinct race of actual vampires although as of now this idea is just conjecture.