I had a werecoyote dwarf locked in small room (for the case I decide to do some SCIENCE with him). Then once I decided to do something with him and remove constructed wall. Shortly I completely forgot about that, distracted by something else. At the next full moon, he of course bit some dwarfs, and I failed again - didn't track everyone he bit. Long story short, finally I had several guests in tavern infected.
At this point, it became too annoying to manage situation on individual level (because I couldn't directly give orders to visitors), so I just set my military ready and grabbed some popcorn.
It have taken about 3 full moons for situation to be resolved. Every single visitor had deceased, and around 40% of my citizens as well (total population dropped to around 30 dwarfs), but infection gone with all its carriers. The tavern looked terrifying: every tile of floor was covered by blood, and a couple decades of corpses were there as well.
If I would play .34 or earlier version, this most likely would end with tantrum spiral and the end of the fortress. So, I'm kinda lucky to be able to continue play my fortress.
Also, funny fact is that I found vampire among dead visitors (I didn't know I had one). Poor naive guy, he maybe thought it's gonna be a resort for him... he didn't even had time to drain anyone (I hadn't noticed any accidents by the time of blood slaughter, at least).