The source of Vampirism is what is know as the Human Vampiric Virus, a symbiotic retrovirus which benefits its host in many ways rather than killing it. HVV is not related to any known genus of virus or viroid. Due to its unique structure and protein-base, it is believed that the virus is of extraterrestrial origin - brought to Earth by either a comet or meteor.
The complexity of HVV is staggering. In many ways it resembles a very primitive virus called a prion. Most viruses cause disease by infiltrating the host's systems then attacking specific cells and replacing the DNA of the cell with a copy of their own thus creating more viruses which then attack more cells and so on. The body of the host then recognizes these infected cells as foreign matter and the immune system works to eradicate them. HVV works in much the same way except that the virus attacks nearly all of the hosts systems and works to adapt rather than change the DNA within the cells, adding extra 'mini-chromosomes' called plasmids to the human DNA rather than replacing it completely. This means that the host's immune system is partly fooled into believing that the adapted cells are still normal and are less aggressive in their removal of them, giving the virus more time to spread through out the body's systems.
Though technically a retrovirus, the degree to which it rewrites its host's genetic code puts it in a league all its own. Most viruses are highly specific in what type of tissue they target. HVV, however, is capable of infecting virtually every cell in the human body.