King Baudouin IV "the Leper" of Jerusalem, 2 January 1177
http://en.wikipedia.org/wiki/Baldwin_IV_of_Jerusalem
Baudouin IV was the famous "Leper King" of Jerusalem, son of previous King Amalric. His fatal sickness was discovered when he was a young boy. According to Runciman: he and his friends were playing one day, attempting to injure each other by driving their fingernails into each other's arms, but Baldwin felt no pain. William ( of Tyre, his guardian) immediately recognized this as a sign of serious illness. Later it turned out to be a leprosy.
In spite of that, Baudouin ruled as a king for 11 years, more than anyone would suspect, even leading armies and making diplomatic agreements sometimes. His sickness didnt give any chance for him to have an heir and stuck him to bed, therefore his influence among people in kingdom was poor, with his sisters and regents having too much power. In the end he wasnt able (or maybe didnt want) to stop the marriage between his sister Sibylla and Guy de Lusignan, so after his death in 1185, the Kingdom was no longer ruled by his dynasty, and in 2 years it was almost eradicated by Guy's poor reign and Saladins power.
In CK2, in 1177, 2nd of January, as a unlucky Baudouin IV, you have just turned 16, but you are seriously sick and having a son would be a miracle. Even worse, Kingdom of Jerusalem is surrounded by most powerfull muslim state it ever struggled with, and in neighbourhood only Byzantine Empire is strong enough to be an usefull ally against Saladin. Not having a heir and being a leper will result in lack of support from your vassals (or maybe even rebellions), not to mention you cant let your sisters marry to other dynasties, because they will most likely inherit the throne after you. Try to achieve as much as you can, before leprosy will enter its terminal stage. Baudouin from real life lasted 11 years as a king and 24 as a man, how long will you last?