In its origins the vampire myth seems to have been very different from modern vampires like Eli (importantly they were decidedly not immortal) :
The anthropologic data collected by folklorists from people who believed in vampires, together with the historic reports, permit a fairly good reconstruction of the legend. Dogs and wolves were the animals most related to vampires, and were also reported as being their worst enemies. A vampire could allegedly turn into a dog and kill all the dogs of its village. Apart from having a human figure, these creatures could appear in the shape of a wolf, dog, or cat, or be invisible. Vampires were usually male, and used to be poor people from rural areas. The vampires' activity was mainly nocturnal, but could be diurnal. Vampires were thought to suck blood and attack people, dogs, cattle, and other tame animals. Vampires were thought to leave their graves to have sexual intercourse. A person could become a vampire by being attacked by a vampire, eating the flesh of animals killed by vampires, having been a great lover, or having died of plague, rabies, or other epidemic diseases. A cadaver could turn into a vampire if it saw itself in a mirror, or if a dog, cat, or bat walked over it before burial. Protective measures used against vampires included rubbing garlic or burning resin, burying suspicious cadavers in islands or lakes, or pouring water around their coffins. The vampires' victims were said to experience suffocation before death. The vampires' life was supposed to last for 40 days. Signs that made a cadaver suspicious included good external appearance, a swollen body full of liquid blood that flowed out of the mouth, prominent genitalia, and the emission of a cry when a stake was driven into it.
I've posted that to me it seems rather peculiar that the vampire myth, which to me seems clearly in its origins at least about death and disease, should have been sexualised so much. According to this article, this apparent paradox may be easily and horrifically explained:
Hypersexuality may be a striking manifestation of furious rabies. Some men can stay "several days with permanent penile erection and even with ejaculations associated with voluptuous sensations." The literature reports cases of rabid patients who practiced intercourse up to 30 times in a day, or who made violent rape attempts.
Something on how cadavers of the rabid could appear 'undead':
Patients with rabies usually live less than 2 weeks and die by asphyxia or cardiorespiratory arrest. These types of death would account for some postmortem features documented in rabies, such as persistence of liquid blood, turgescence of the genitalia, and emission of sperm.
And some real-world evidence of how people actually respond to vampire-like diseases:
People with this disease, however, sometimes faced a more dramatic end: "The fear of rabies was such that often persons even suspected of hydrophobia were killed like wild animals . . . (by being) shot, poisoned, strangled or suffocated."33 The social alarm prompted by rabies moved some medical academies, religious institutions, and kings to propose remedies.
Other phenomena that probably contributed to the myth:
In 1985, Dolphin suggested that porphyria might have been the basis for the vampire legend 36; his unpublished theory has been criticized elsewhere.37 Kayton supported a connection with schizophrenia, arguing that "many behaviors and inner experiences of schizophrenics are similar to aspects of the vampire legend."38 Barber 18 attributed the belief to the fact that laymen are prone to misinterpret the observations made on cadavers. Thus, the idea that vampires sucked blood would be based on the presence of liquid blood inside bloated corpses and at the corners of corpses' mouths. Barber hypothesized that dead bodies might have been disinterred by dogs, wolves, or spontaneously due to superficial burials in times of epidemics. This would explain, in his view, both the idea that vampires left their graves and the implication of animals in the legend.
The concluding paragraph:
Much evidence supports that rabies could have played a key role in the generation of the vampire legend. This would be in accordance with the anthropologic theory that assumes that many popular legends have been prompted by facts. Under this approach, saying that the vampire is "mere fiction" may be somewhat inappropriate. The past occurrence of epidemics of men and animals that bit others and transmitted the same disease to them can now be scientifically understood. It can also be accepted that those men showed hypersexuality, intolerance to mirrors and smelling substances, and liquid blood after death. Finally, it can be scientifically stated that this unique picture may be seen, even nowadays, in some unfortunate cases of furious rabies.