According to "Vampires, Burial, and Death" by Paul Barber, the typical vampire before the 19th century literary craze was a peasant who had died an untimely death and/or had been difficult in life. The thesis of the book is that the folkloric tales of vampires were essentially accurate descriptions of the effects of an untimely death. The key thing is you have to ignore the explanations for the events in the story and focus on the events themselves.

Here's the sequence sequence of events: a person dies suddenly, perhaps violently. They bury the person, but a little while later people think they start seeing him (or her) as you do sometimes after a person has died. Bad things start happening and instead of blaming a witch they start blaming the dead person (who may have been considered a witch or werewolf in life if they were troublesome). This could be mischief level stuff, or it could be more serious: crop failure, livestock sickening or dying, or other villagers dying--this last was particularly likely if the "vampire" was killed by an infectious disease. After they decide the dead person is responsible for the troubles the natural thing is to go to the burial site to investigate. Perhaps they see the soil is disturbed around the gravesite, which would be an obvious sign that the person had left the grave. (The real reason could be that wild animals or dogs had smelled the corpse and tried to dig it up, or that the coffin had collapsed, or even the dirt had simply settled). Whether or not the dirt was disturbed, they might then decide to dig up the corpse to see if there were any signs of it being a vampire. If they didn't find a corpse (because an animal had dragged it away perhaps) the vampire was out walking the earth (folkloric vampires can go out during the day). If they found the corpse they might be surprised by the state of preservation--people who died young of quick causes will often take a lot longer to decompose than you'd expect. They also may notice the person looks bloated as if they've been sucking blood and they may even have blood on their lips. This is typical of bodies that have gasses from decomposition bacteria in their body cavities. If they touch the bodies they may notice that they are not stiff with rigor mortis (rigor mortis doesn't last forever) and if they cut them they may see liquid instead of clotted blood (again this is normal, clotting won't last forever). They may see the body has moved from its original position. This could be because the aforementioned gaseous buildup caused the body to shift, or gravity and jostling. If they decide to stake the corpse--only one of many ways to deal with a folkloric vampire--they might hear a groan and see the vampire struggling. The groan would be from air being forced from the lungs, and the struggle would be the physics of a body being whacked with a hammer and stake.

Tl,dr: vampires as depicted in folklore (rather than fiction) are the result of accurate observations of what happens to a body (and a nearby community) after death, just with incorrect reporting on the causes.

Edited: 10th century now 19th century.

(First, a little background. If you want to skip right to the meat of this comment, scroll down a bit to the paragraph about ergot. Click the links; I'll add some sources at the bottom as I find my books.)

As long as humans have existed, we have sought power from beyond, from arcane or ethereal sources. We enjoy the escapism, we enjoy the idea that anyone could perform the right ritual and obtain significant power, right at our fingertips. This has given rise to many myths, stories of people who have acquired the strength of the bear, the silent prowess of the jaguar, the cunning of the fox, and the power of the wolf. Most of our oldest werewolf origins come to us from Greek, Norse, or Celtic accounts, which makes it difficult to determine a specific event or origin. Shapeshifter stories come from all over the world, on every continent, and over time they tend to weave into each other and influence one another.

In the Western world, many of the recorded means of becoming a werewolf are usually a way of subverting the church, which goes hand in hand with the idea that lycanthropy is attached to pagan or Satanic practices, and therefore evil. Sometimes, lycanthropy is attributed to demonic possession, and is therefore the church's domain to treat, or to drive out the wolf-demon from the afflicted. (Similarly, the most famous way to kill a werewolf features silver, a notably "pure" and "untainted" metal, also common to churches, as they could afford such luxuries.)

For example, one such method of becoming a werewolf is to place a special wolf skin belt over the entrance of a church at a wedding. Depending on whether the belt is fur or skin side up, it will turn all of the wedding guests who pass through the door into werewolves, often without their knowledge or consent. Traditionally, the Catholic church has several established sacraments or rituals that follow their faithful throughout major life milestones, such as birth (baptism), childhood (Holy Communion), starting a new family (marriage/Holy Matrimony), death, etc. In this way, this particular method could be seen to subvert one of the most defining moments of a young adult's life, when they are becoming independent and starting their own family.

Similarly, many of the "recipes" for potions or rituals that claim to impart lycanthropy upon the imbiber or the participant feature things that we know in modern times to be poisonous or hallucinogenic. Ergot, in particular, but more on that in a moment. For example, some of the ingredients include hemlock, or unwashed heather. (This is also an ingredient in several sacred beers, as unwashed heather usually harbors ergot, which is hallucinogenic.) It is not uncommon for several mild or readily-available hallucinogens to be present in these potions. Sometimes they are also burned or smeared on the body in a paste during rituals.

Ergot: Ergot is the likely the true culprit behind a lot of werewolf stories and myths. Ergot poisoning, or ergotism is a form of food poisoning that is prevalent in various grains, especially rye. It's not native to the rye, mind you, it's actually the result of a specific fungus, Claviceps purpurea. Once afflicted, the side effects include

"...painful seizures and spasms, diarrhea, paresthesias, itching, mental effects including mania or psychosis, headaches, nausea and vomiting. Usually the gastrointestinal effects precede central nervous system effects."

The Greeks were known to use ergot for its hallucinogenic properties. For example, in the Temple of Eleusis, supplicants would fast prior to entering the temple. If breaking their fast, pilgrims would use a potion called kykeon, a purple potion containing ergot. The Temple of Eleusis was also devoted to the goddess Demeter, who presided over crops and the harvest.

Ergotism also causes mania, convulsions, and causes the body's blood to move towards the core of the body, giving rise to ergotism's archaic name, St. Anthony's Fire, due to the increased body heat. Along similar lines, it also causes the hands and feet to curl up, much like an animals' paws. The gangrenous symptoms cause the afflicted's ears, nose, hands, and feet to become dark and foul-smelling. Imagine: your neighbor is raving and convulsing, he can't stand on both of his feet and must go on his knees or on all fours, his hands are curled like a dog's paws, and his body is hot to the touch. Must be a werewolf, right?

There are also reports of starving farmers going mad and turning to cannibalism; again, this may be due to ergot poisoning or it may be due to starvation. If your annual crop has failed, you are more likely to eat the "tainted" grain. It's still good, right? It just tastes a little funny. If you get enough into your bread or if it infests the local mill, then your community is going to have a problem.

You may also enjoy this academic paper about the historical basis of lycanthropy. There are also genetic diseases which cause the teeth to become more prominent or cause body hair to sprout very thickly all over the body; most of our modern-day "wolfmen" such as Jesús Aceves, may point to a historical origin for tales of werewolves. It would not be unheard of for someone with one of these to find themselves living on the outskirts of civilization.

I am not a historian but rather a physician. There is a controversial belief among doctors that there is a medical basis for the vampire myth, which was largely propogated in medical journals in the 1960s by several doctors, the first of which was L. Ellis in the Proceedings of the Royal Society of Medicine titled "On Porphyria and the Aetiology of Werewolves" linked HERE. Of note this has largely been discredited but it still may add to an interesting discussion.

In short: there is a cluster of medical conditions called porphyrias, which are essentially heritable hematologic abnormalities which involve enzymatic deficits in the production of heme (the ferrous component of hemoglobin, the oxygen carrying molecule in one's blood). There are cutaneous porphyrias which involve intense photodermatitis that may involve painful bloody blisters when one's skin is exposed to sunlight, which may exacerbate the omnipresent anemia present in those with porphyria of any type.

Porphyria also can produce unusual behavioral changes, and severe anemia can cause unusual pica or other eating disorders, some of which may involve eating/drinking the blood of animals, and in the case of anemia induced pica, various minerals or even dirt.

Another medical culprit could be acute rabies infection, which is fatal, causes sleep wake cycle abnormalities (people can become nocturnal), hydrophobia, and may cause aggression and various neuropsychiatric phenomenon when advanced.

I don't have any information on actual events, but I know of an interesting tidbit about werewolves. The earliest known mention of what we know as werewolves was first mentioned by the Roman author Petronius in the late first century CE. The entierty of the work is called Satyricon, but werewolves show up in the section of the text called Trimalchio's Dinner.

The story is about a freed slave who has accumulated mass amounts of wealth and throws these lavish parties. At the parties, wild stories are told to entertain the guests. The mention of the werewolf comes in chapter 62:

"It so happened that our master had gone to Capua to attend to some odds and ends of business and I seized the opportunity, and persuaded a guest of the house to accompany me as far as the fifth mile-stone. He was a soldier, and as brave as the very devil. We set out about cock-crow, the moon was shining as bright as midday, and came to where the tombstones are. My man stepped aside amongst them, but I sat down, singing, and commenced to count them up. When I looked around for my companion, he had stripped himself and piled his clothes by the side of the road. My heart was in my mouth, and I sat there while he pissed a ring around them and was suddenly turned into a wolf! Now don't think I'm joking, I wouldn't lie for any amount of money, but as I was saying, he commenced to howl after he was turned into a wolf, and ran away into the forest. I didn't know where I was for a minute or two, then I went to his clothes, to pick them up, and damned if they hadn't turned to stone!"

I had the opportunity to read this when I took Latin at uni and found it fascinating. Werewolf pee turns things into stone. I'm sorry if this doesn't help you find an answer, or is off topic, but the literature is interesting!

A question related to this question: could rabies have anything to do with the zombie myth? I mean, they turn mindless, start wandering slowly and then attack people, and if you get bitten you're infected etc.?