The vampire disease you can catch over and over, turning into a vampire. The one that makes you a werewolf was a one time deal seeing as how it took a quest line to catch it and a line to cure it.
I hope that changes with dawn guard and the introduction to werewolves in the wild --- It's a hobby. If the thing you go hardest at in life is video games, you don't get to call yourself hardcore. You're soft, and should be treated as such.
Not sure, as I haven't followed through with it yet, but Aela (who I married in-game; doubt that matters though.) offers to let you regain the blood of the beast.
You get an "Are you ready?" dialog before setting off, but I plan on coming back to it when I'm done with DG. Unless that's a huge ruse, I imagine talking to the Companions, or in my case at least, Aela will get you started. --- I make video games.