According to Lucinda Szabo in the story "Sweet Blood", this is how vampires may have come to be:

It started off with a diabetic person that was untreated because medicine had not advanced that far yet. "The first thing is she starts getting very hungry and thirsty. She can't get enough water. She devours bowl after bowl ofgruel (whatever that is). At first her parents are angry at her because thay are poor and gruel is not free.But she can't stop herself from eating everything in sight. Soom she starts loosing weight. She eats like a pig but the food is going right through her. Her parents are afraid she might be possessed. They hide her from the neighbors because if word gets out, their daughter could be burned at the stake. Weeks go by. the girl has lost a quarter of her body weight. She is pale and smells sweet, like honey. She sleeps most of the day, but it is a restless sleep, tossing and turning and whimpering. When she awakens she is hungry and thirsty. She wets the bed repeatedly, and after a week of that her mother stops bringing her fresh straw and simply lets the girl lie in her own filth. The girl doesn't seem to care. She talks to herself in her sleep, crazy garbled conversations with imaginary people. Her skin becomes pale and beaded with sweat, her lips are ruby red, and she has a peculiar, acrid odor.One day the mother brings the girl a chicken leg. The girl sits up on her soiled straw pallet and snatches the chicken leg and tears into it with the ferocity of a starved wolf. The mother recoils from what she sees-the girls teeth have grown longer, and her mouth is bloody. She gobbles down the chicken leg, crunching the bone between her long, bloody teeth. Her breath reeks like a stew of rotten fruit and fetid meat; her eyes are so dialated that they look like black holes in reality. the mother, terrified, flees. The next day, the girl staggers out of the cottage looking for food. As the midday sun strikes her she screams and covers her eyes and crumples to the ground. Her parents are shocked to see her this way, in full sunlight. Her skin is white as a fishe's belly, her hair has fallen out in patches, her linbs are thin as broom handles. The father carries his waisted daughter back to her pallet and lays her down. The next morning the girl is still and unresponsive. Her forehead is icy cold. There is no sign of life. The mother tells the father that the girl is dead. They cover her with a sheet. They tell the neighbors that their daughter has died. Tomorrow they will bury her in the graveyard by the village church. But that night a strange thing happens. The girl awakens. She throws aside the sheet and climbs to her feet. She does not know where she is, but she is ravenous. Shr staggers through the cottage, confused and terrified. The mother sees her and screams on horror. The girl claps her hands to her ears. Too loud! The terrified father grabs a knife and waves it ar her. She runs from the cottage, runs form the screaming and the flashing blade. She sees a flickering light in the distance. She hears voices. She smells cooking! She heads for the light, bursts int her neighbors' cottage, grabs a chunk of pork from their stewpot and crams it whole into her mouth. The neighbors run away and the girl gobbles their supper. She wanders off into the countryside. The next day some village boys find her lying motionless in a beet field. The village elders are called. The local tooth-puller pronounces her dead; the priest says that she is possessed. They decide to burn her quickly. A pyre is erected on a hilltop. The girl's body is placed atop the enormous pile of dry logs and branches. The priest throws a torch onto the pyre and within a few seconds the flames are roaring and the villagers' faces are orange with reflected firelight. Then something inside the tower of flames moves, and they see the shape of the girl. She erupts screaming from the pyreinto their midst, her entire body on fire. She twists and turns and leaps in a dance of death as the villagers run shrieking. Then she dies- for real this time." All the symptoms are posible symptoms of untreated diabetics. "The sweet smell of too much glucose in the blood, the strange, acrid reek of advanced ketoacidosis, the rotten smell of bacterial infection. Madness, ravenous hunger, extreme sensitivity to sunlight and sound, bleeding, receding gums (that make her teeth look longer), cold, clammy skin, and deathlike coma. even spontaneous, repeated revival from a deathlike coma is possible. "