Blood is a red fluid that transfers oxygen, nutrients, and metabolic waste around the body of humans and other animals.
Vampires must drink blood of the living to stay alive. Fledgling vampires must drink blood (or a blood substitute) or their mortal body will die, but when a fledgling drinks human blood they turn into a full fledged vampire. In Blood Drive it's revealed that Ethan has H-deficient blood, which means it's considered by vampires to be the best blood there is (because only one in a million people have it), thus making it taste and smell particularly delicious to vampires. In ReVamped, it's revealed that vampires and fledglings can suck the venom out of a human to prevent them from becoming a vampire, much like one can remove snake venom from a person's body.
Types of blood
H-deficient is a rare type of blood that Ethan has. Vampires love it; they think it is refreshing and smells particularly delicious. They consider a pint of this blood is worth all the blood in Red Riders's van. It is known that only one in a million people have H-deficient blood.