Porphyric Hemophilia

From the OblivioWiki – your guide to everything Oblivion

Porphyric Hemophilia is the vampirism disease. There are two 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.

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 the three days time has passed.

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

For more information on being a vampire, see vampirism.