Blood
Blood, blood everywhere and not a drop to drink...
It's the blood that makes life possible. When humans are alive the blood flows freely through our veins, when we die the blood stops flowing. If we are cut the blood will spill from the wound. If we lose enough blood we will die. For ancient cultures life was as simple as circulating blood. They had no real science so they only knew what they observed. If we have blood we stayed live, if we lost blood we died. Therefore blood must equal life itself. Ancient civilizations made blood sacrifices to their Gods and warriors drank the blood of their enemies for a number of psychological reasons. Even today in the Catholic Church they drink from goblets of wine or juice that symbolizes the blood of Jesus Christ.
Vampires are themselves dead. No blood flows through their corpses. To keep themselves in an undead state of existence they must steal the flesh blood from other living creatures. It doesn't do vampires any good to drink the blood from dead creatures, because it's the "lifeblood" that the vampires needs.
Vampires cannot get common blood diseases. Again, vampires are not human. They are supernatural creatures of darkness. They feed on the living to steal life. They are immune to human fluid diseases such as AIDS, Hepatitis, Rabies, etc. Vampires cannot die from disease. They are already dead. Vampires are in essence parasites, (they need hosts to feed from) and like most parasites they are resistant to the bad bacteria and any disease that might be present in the host's fluid.
The blood of a vampire is different from living blood. Once a vampire consumes the blood from a living creature that blood becomes their lifeblood and is tainted with vampirism. Any human who drinks vampire blood will have their own blood slowly contaminated by the vampirized blood until they too become a full vampire. Recent vampire history states that if a person survives a vampire's bite that they will slowly transform into a vampire, but this wasn't always the case. In traditional vampire lore the bite alone could not change a mortal human into a vampire. Anyone who changes into a vampire must first go through a human bodily death then become reborn in undead form, forever needing fresh lifeblood to stay animated.