I'd wondered why people back then thought it was vampires. Well, in 2006, a 16th-century woman was unearthed in a mass grave near Venice among some graves of plague victims. The woman also had a stone in her mouth, a "vampire".
But why did the people during the Middles believe in vampires? Well, apparently the "first recorded use of the term vampire was to refer to a Russian prince that scientists now believe may have been suffering from rabies. At some point the Bohemians switched to driving a stake through the hearts of vampires, but in the early period burials with a stone in the mouth were the accepted 'cure' to prevent them coming back to life."
During early burials, vampires were "recognized" probably because of decomposition, a phenomenon not understood back then. When the body decays, the stomach releases a dark "purge of fluid". Bloodlike, this liquid can flow from the corpse's nose and mouth. Often the burial grounds were reopened during the plague to add new bodies, a way to recycle graves. Gravediggers would have seen the decomposing remains and may have been confused the purge of fluid for the blood of the vampire's "victim".
Not only was there this "purge" that they thought was evidence, but sometimes the shroud near the corpse's mouth was torn, giving the impression that the dead was chewing through the cloth. In reality, the fluid expelled from the body sometimes moistened the shroud, causing it to sag into the jaw.
Vampires were thought by some to be the causes of plagues, and the superstition took root that shroud-chewing was the "magical way" that vampires infected people, Borrini said.Inserting objects, then, such as the stones, into the mouths of alleged vampires was thought to halt the spread of disease.