The belief in vampires is one of the most wide-spread of superstitions with "real-life" instances of vampirism having been documented for hundreds of years.
How can all those eye-witnesses to these creatures of the night be wrong? One answer modern science gives us is that they weren't all wrong: a lot of the written accounts of vampirism are accurate descriptions of events that actually took place. It's just that the interpretation was flawed.
The majority of documented proof about vampires surround recently exhumed bodies. They weren't people seen sprouting fangs, turning into bats, or actively drinking blood: the evidence is based upon how the already dead appeared.
How The Dead Were Seen
They were diagnosed as vampires because of "proof" such as not having decomposed enough or there was what looked like fresh blood surrounding their mouths.
This appears to be a self-reenforcing myth. In the days before embalming, bodies were simply buried in porous wooden boxes. Under these conditions, natural decay causes a gradual bloating due to the release of gases. It may even cause blood to emerge from the corpse's mouth as the lungs are squeezed.
To top things off, driving a stake through the chest of a body in this state can cause it to emit an eerie cry. Not because the person is still alive, but because gases are being forced outwards from the force of the stake.
Other theories for what people confused vampirism with is diseases. Plagues, rabies, tuberculosis (also called the great white plague), and even mental health issues such as schizophrenia, may have been confused with vampirism at different times.
Barber, Paul. "Forensic Pathology and the European Vampire." Journal of Folklore Research 24, no. 1 (1987): 1-32. (You'll need access to JStor to read this article)
Johnson, Eric Michael. "A Natural History of Vampires." Scientific American. October 31, 2011. Accessed November 14, 2016.
Want to read and think about a completely opposite point of view on vampires? Read:
Browning, John Edgar. "Real-Life Vampires Exist, and Researchers Are Studying Them." Discover Magazine. March 26, 2016. Accessed November 14, 2016.