Welcome!
And, I think it actually got mentioned in the Umbral/Astral magic debate earlier... but I think it had something to do with the fact that Arcanima (the magic Arcanists/Summoners/Scholars use) does not use elements like Conjury/White Magic or Thaumaturgy/Black Magic. It's "unaspected" aether.
And so... somehow that turns into pathogens? Something like that. However, I don't know if there's actual lore in-game that references this stuff or if it's extrapolation based on theories by the players.
Thanks for the response!
Given some of the high-level spells are clearly guided expansions on the basics such as Fester (it doesn't seem to be something you just 'stumble on') is it possible such specialization is intentional? At the very least it's a pretty brutal way to kill someone even compared to the direct elemental magics, and nobody seems to bat an eye on an IC level about it.
It's certainly an interesting angle but I'm also not sure how it'd tie into general Arcanist work.