Other plague-like symptoms should show up, if only to make it more recognizable as a sickness, and not just an oddity. Weakness, general lethargy, achy sensations etc.
I was thinking close contact - a touch, being withing close speaking/breathing distance, the normal things that spread a cold/flu etc - might spread it, maybe from person to person with at least one talent in common, and that's the one that gets affected? Or something less complicated maybe...I tend to over-complicate sometimes.
That make sense?
There's probably a patient zero that first contracted it though and the reason they did could be a contaminated object or a wizard experiment gone wrong, just so there's something tor trace it back to and thus find a cure.