Porphyric Hemophilia

From the OblivioWiki – your guide to everything Oblivion

Porphyric Hemophilia is the vampirism disease. There are three ways to acquire this disease, which can bring you many combat-related benefits, as well as a few curses. The first is fighting with a vampire and contracting it as you would any other disease. The second way is through the Dark Brotherhood faction. After reaching a certain rank you will be offered the chance to become a vampire by Vicente Valtieri, but if you decline you can always ask again later; although, if you continue to complete the Dark Brotherhood questline, you will lose this option as you will have to kill Vicente Valtieri.The third is by eating the beating heart during malhunes razor add on

Poryphyric Hemophilia also drains your Fatigue by 5 points. If infected and you wait for three days, the next time you sleep you will wake up a Vampire. If you do not wish to become a vampire, simply cure the disease -- by praying at a temple, or using a potion or scroll -- before you're infected with Vampirism. Even if more than three days has passed, you can simply cure yourself, given that you haven't rested.

Even though it can be contracted more than once, the vampirism will only happen once.

For more information on being a vampire, see vampirism.