I wonder how loudness factors into that. Maybe ideas which make their "host" express them loudly have a higher chance of "infecting" others?

If you haven't already, you should check out Daniel Dennett's "From Bacteria to Bach and Back". I think you'll be susceptible to "catching" a lot of the ideas in there :)

Or express them at all. Ideas which when heard are likely to be repeated, loudly. ...We're basically describing idea zombies, a la Pontypool