The idea that Noxiphilic Sanguivoria generally requires a blood transfusion to turn you into a vampire is absolutely wrong. This is only true if the subject is a soulless one. Here is what Rahaja says in the vampirism quest, after the player has simply been bitten: "If you were like other mortals you would soon turn. But you are unique. The Dark Father, Molag Bal, has corrupted you. Only Mother Lamae herself may turn you into one of us." This is also compatible with what Cinna Scholasticus writes in his book Noxiphilic Sanguivoria, as he states that "Some of them have described entering a ritual chamber where they were bathed in a pool of black blood". Some, not all. So the disease itself does not generally require a ritual for the subject to be turned into a vampire. The blood transfusion ritual is required for the Rite of the Scion, whereby a simple vampire may become a more powerful one, or in order to turn a soulless one into a vampire (also a Scion, presumably).