At some point in your gaming career you will probably be infected with either vampirism or lycanthropy. At some point thereafter a letter will show up offering you the choice of seeking a cure or being hunted.
Lycanthropy: the letter launching this quest just kind of appears in your inventory at some point after you contract the disease. It is dated the 1st of Morning Star, but arrives at a random time (kind of like the real postal system, huh?). You will be directed to a contact in a temple who will then direct you to an alchemist who draws a little of your blood. You must then go find another person. This person is always dead, but you are directed to yet a fourth person (a kid) who drinks the blood and changes into a werecritter and attacks you (giving you the opportunity to contract the disease again -- sheesh!). As soon as the kid drinks your blood, you are cured.
My conscience kind of nags me on this one because it's a kid, so I very seldom follow up on this quest. Usually I will either endure the hunters (they show up every couple of nights) or seek out a Coven for the universal cure. The Hircine Ring makes lycanthropy very easy to live with, and the hunters occasionally have decent goodies.
Vampirism: the letter launching this quest appears randomly, but it isn't dated so I can't make any snide comments about the postal service.
You are directed to a particular vampire to get the name and location of your "bloodfather". If you show the letter, you will not be given the name of your quarry. If you do not show the letter, you will be given two names and two locations. The second name and location is the correct one. You will also be given the name of a another vampire in another location who can tell you the correct name and location and a bracelet to identify you (don't lose the bracelet). If you kill the wrong vampire, the real one will get away and the rest of the tribe will be on the alert for you. Your "bloodfather" is a Vampire Ancient, so don't undertake this quest lightly. Vampire Ancients are tough critters, even for high-level characters.
If you choose to not follow up on this quest, you will get a Knight every couple of nights that tries to kill you (can be a good source of goodies). If the curse of vampirism gets to be too much for you, you can always go seek out a Coven and get the universal cure. I think Bethesda should have let PC's become hunters and tossed in a couple of Knight or Fighter quests to liven things up. Maybe in "Morrowind".