Medical: vampires are just people affected by rabies.

People with rabies become adverse to strong odors (like garlic) or stimuli, like watching himself in the mirror. Biting of course is part of the symptoms and the disease is transmitted through bites.

After death it's common for these people to have blood oozing from their mouth, as if they bite someone just before.

And guess what nocturnal animal is carrying rabies ? Vampire bats.