Very well. Incredibly, incredibly summarized to an almost insulting degree: Before humanity's era, enormous and terrible Gods known as the Great Ones lived. We are still finding their remains and signs of their presence, and it was discovered that the use of the Great Ones' blood could heal via a blood transfusion. Naturally, everyone who was treated was. Unfortunately different strains produced different results, some altogether dangerous. The Church itself controls different strains of the blood and regulates what reaches the markets, but there are always those who use different strains that have been declared impure.
Anyway, blood ministration is an altogether pleasurable thing, if very dangerous if not used in moderation. Like drinking, I'd imagine. Most of those who work the streets have strains that have been condemned by the Church, but are nonetheless in high demand.