I have this vague memory of having seen a character that could/would dump his own Flaws onto others nearby. He's Jinxed and you interact too much with him? Now you are Jinxed too!
I can't remember whether this was the result of some Flaw, some supernatural ability or power, or something else; and whether this was a canon NPC, a character built according to some obscure canon rules, or something completely made up. Any pointer you could provide, however, would be greatly appreciated!
I do know it exists in other game systems, but I dont' think I've ever seen it in Ars Magica - in GURPS (3rd edition) that disadvantage existed. It was either Jinxed or Cursed, but it basically gave everyone BUT your character a penalty to all rolls (I think...maybe only hidden rolls by the GM? I forget). It was one of the disads that no sane PC would take, as the first thing people would do once discovering it would be to kick you out of the party and demand that you create a new character.
There's the "Contagious Obsession" power that some demons (Vessels of Iniquity) have - see RoP:tI page 39. That's limited to just being a contagious version of a standard Obsession power rather than affecting any virtue or flaw, though.