'Tain't a purty picher, but there ARE diseases that could have given rise to the vampire (vampyre, wampyre, etc.) legends.

Here's one place. Here's a more authoritative one.

In any case, the vampire of legend, of Dracula, doesn't and can't exist. But humans HAVE use blood -- horse, sheep, cow, pig, human, and other varieties, for food. Drisheens, black pudding, blutwurst, and the purported practice of the Mongol Hordes drinking the blood of their horses are all examples.

Shucks, you ingest your own blood every day.

Dracula-type vampires certainly wouldn't be popular if they did exist, always sticking their friends for the drinks and all. And geez, what sort of guy wouldn't like a big ol' stake dinner? (I think I'll quit right now before it gets out of hand and I find that my research has been in vein.)