|Jiang Shi i.e zombie in China|
So who would want to consume blood at all?
The famous Count Dracula of Romania is widely considered to be the most popular vampires of all. He was a Count, a nobleman in Romania during the dark ages, and he sold his spirit to the Devil so that he could achieve immortality. But he needed constant input of blood for sustenance.
Alexander Corvinus was a Hungarian warlord from the dark ages. He had two sons, Marcus and William Corvinus, Marcus was bitten by a bat and became the first vampire, and William got bitten by a wolf, survived and became the first werewolf. It was said that the bubonic plague that wiped out one-third of the population of Europe at that time came into his place and killed everyone except Alexander himself. He was baffled by his own ability to survive the plague, not knowing that he was achieving immortality. And this immortality got passed down to both of his sons.
Vlad the Impaler, nicknamed Vlad Dracula.
He was a Prince of Wallachia, and his favourite punishment was impalement. It was said that minor offences were also treated with impalement. There was a lady who had an affair got her breasts cut off and later skinned alive (Chinese also skinned people alive during the Qing Dynasty), and then was impaled at the centre of the square, with her skin on a table next to her pole.
Vlad's involvement in war also meant that he impaled a lot of soldiers. The mighty Ottoman army that wanted to have war with him withdrew their soldiers when they arrived at the bank of Wallachia because the soldiers were so terrified by thousands of corpses of their dead comrades on poles on the river bank.
The Blood Countess, Elizabeth Bathory was also from Hungary. After the death of her husband she involved in the torture and killing of young women, with one witness attributing to them over 600 victims.
Nobody knew why she was acting this way, though there were later writings claiming that she bathed in the blood of virgins in order to retain her youth.
Non-fictional vampires aren't human and they aren't immortal. Blood is not a good source of food so it's a pity they have chosen blood as their sustenance. The primary function of blood is to transport oxygen and carbon dioxide in our body. Apart from that, it also transports toxin into our kidneys to be filtered, and transports nutrients to various part of the body. An average human have about 5 liters of blood.
And since blood is of such low quality, why would anyone make it as their primary source of food? The vampire bat for example, has to drink a lot of blood to survive.
For smaller animals, their relatively small metabolism enables them to thrive on blood. Besides, blood is an easy food source, every living being has blood. And if we are to live on blood, we have to drink a bathtub full of blood everyday in order to survive.
Evolution does not allow human to thrive on blood alone. Enough said.