>Kan-E-Senna even mentions this at the end of the Gridana starting storyline

I started Conjurer, I'll have to go back to my Inn Room and see how exactly she phrased that - been too long for me to remember. It may be a catch-all term for any seeming sickness which is not well understood. That would certainly explain why our Archon friends end up asking us if we experienced anything like Aether-Sickness in order to confirm that we did, in fact, have The Echo.