h2g2 The Hitchhiker's Guide to the Galaxy: Earth Edition

I read somewhere (including the excellent novel "An Instance of the Finger Post" by Iain Pears) that until fairly recently it was believed that the soul was the life-giving property in blood. If your blood ran out you died and if you consumed young blood or the blood from a strong animal (say ate bloody meat from a bull (even though cooked)) you gained the qualities of the blood you consumed.

It makes sense that vampires lived off human blood and thereby gained spiritual sustenance by essentially drinking the souls of their victims.

Yummy.

There is another perspective - as far as I know there is a blood disease (I don't remember the name but I can check if you're interested)which in ancient times was thought to be cured by drinking blood. Blood infusion was not yet known and since blood can easily be absorbed through the walls of the stomach, drinking it made the patients well again for a brief span.

The diase was called pophoria, which made the sufferers exceptionally hairy and sensetive to light, thus giving rise to the werewolf myth, an also the general association between werewolves and vampires.

A lot of people believe(d) that vampires are primarily spectral entites, leaving their coffins in astral form to feed on the life force of the living. This may have gotten confused with blood at some point.

I wouldn't have thought that many people would have considered the possibility of an Astral form until quite recently. The fixed ideas of heaven and hell as physical places existing relative to the earth'ss surface pretty much dominated cosmological speculation until the 19th century. Ghosts were real enough though.

The connection between cells and mechanisms for healing took off after the discovery of petro-chemical dyes (extracted from coal) in the mid 19C. Until then the stuff of "livingness" (the soul) was a mystery and medical practices tried to deal with making people live better rather than physical ailments.

Confusing life force with blood was an obvious mistake to make.