Hematophagy is the habit of certain animals of feeding on blood (from the Greek words, haima, blood, and phagein, eat). Since blood is a fluid tissue rich in nutritious proteins and lipids and can be taken without enormous effort, hematophagy has evolved as a preferred form of feeding in many small animals, such as worms and arthropods. Some intestinal helminth worms, such as the Ascaris, feed on blood extracted from the capillaries of the gut and about 75% of all species of leeches (Hirudo medicinalis), a free-living worm, are hematophagous. Some fishes, such as lampreys, and mammals, especially the vampire bats, also practice hematophagy....

Drinking blood and manufacturing foodstuffs and delicacies with animal blood is also a feeding behavior in many societies. African Masai mainstay food, for instance is cow blood mixed with milk. Blood sausage is eaten in many places around the world. Some societies, such as the Moche, had ritual hematophagy, as well as the Scythes, a nomadic people of Russia, who had the habit of drinking the blood of the first enemy they would kill in battle. Some religious rituals underline the importance of metaphorical hematophagy, such as in the representation of blood of Jesus Christ by wine during Catholic mass. Satanic sects in the West have been reported to drink human blood from willing donors and psychiatric cases of hematophagy as a symptom also exist. Finally, real or imagined, human vampirism has been a persistent object of literary and media cult, and tales of blood-thirsty Count Vlad, the originator of Dracula, haunt the masses.

from "Hematophagy" at Wikipedia