Living their life as either a magician or an immoral person, committing suicide, or even having an animal jump over the corpse is enough to turn a person into a vampire as believed in Slavic folklore.
In South Slavic folklore, it was believed that vampires go through several stages. The first 40 days involved starting off as an invisible shadow, and then gaining strength through the blood it sucked. It would then turn into a "jelly-like, boneless mass" before building up a human like body. He would be sexually active and could even have children with his widow, or a new wife.
To combat these demons, twin brothers would use twin oxen to "make a furrow" around their village. In the house of the recently deceased person, an egg would be broken and a nail driven into the floor. At the cemetery the evening after the funeral, elderly women would stick five old knives into the grave; one in his chest and four in his arms and legs.
It is also believed that they are able to turn into butterflies and fly away.
And you wonder why I don't like butterfly houses.

