Vampire mythology goes back to at least the 12th century (Serbia), according to what written records have survived. It’s likely that verbal versions of the legend extend much farther back. What could the origin be?
In the middle ages, vampirism, witchcraft, and other tales were believed to be the explanation for plagues. One person would get sick and die, and their “curse” would kill their families and friends. However, this was not the origin of the vampire myth.
Before that, there were legends of people who would feed on the flesh and blood of others. These were referred to as vampires (by various names), and many myths were created around them. I wonder if this is the true origin of the vampire myth.
In more primitive times, when food was scarce, one survival strategy would have been to find people who were surviving and use them as a supplemental (or sole) source of nourishment. Agronomistic societies–those who farm and organize into settlements/towns/cities–would have been likely targets. A human predator would have used the blackness of night to stumble into town, find a victim, kill them, and–wasting nothing, including blood–get what nourishment they could. Sometimes, another person would have interrupted the act and found the predator first drinking blood from the dying victim, either as a meal in itself, or as a precursor for further predation.
In ancient legends, vampires were commonly female. A woman who was alone and not part of a settlement, at a time when physical strength was paramount to individual survival would have been at particular risk of starvation, and more likely to attempt such a desperate act. Since such a person would likely not be able to overpower a healthier, better-fed man in a fight, seduction would have been a primary means of initially pacifying a potential victim.
A person who fed in such a fashion would be sickly, as there are not only pathogens to be contracted via the consumption of human flesh; but someone who subsists largely on a diet of probably-skinny humans will become severely malnourished, in time, as a result of protein poisoning. A sickly pallor, a slow heart rate, and other symptoms would have given rise to the classical appearance of vampires.
I submit that the origin of the vampire myth is cannibalism, primarily as a result of food scarcity.