Sanguinare Vampiris

Sanguiare Vampiris is the disease that leads to Vampirism this disease is only present in Skyrim, the disease that lead to Vampirism in Oblivion was called Porphyric Hemophilia. It is contracted by engaging in combat with a vampire, every time the vampire hits you with a spell of weapon, you have a chance of contracting this disease.

An easy place to contract the disease is Broken Fang Cave.