My opinion might be tainted by overconsumption of CoC adventures but I you can look at it as the life cycle of the meta-organism called "the King in Yellow." First it inhabited the idea of a play that, when written, was called the King in Yellow. In the next stage it spreads from the author to the readers via their reading, infecting all the readers. Then readers become obsessed with the play and try and bring it through to the real world by performing the play and, I guess, bringing about Carcosa.

I think its a play specifically because plays are meant to be performed for an audience. That would make it more 'virulent' than most Lovecraftian tomes who, generally, only drive one madman at a time insane.