SMN spells are, for the most part, based on diseases. It has been a long time since I went through the story, Can any one direct me to information on why SMNs have this theme for the spells?
Best I can come up with on my own would be...
Summoning Egi requires manipulating biological matter, a skill that lends itself to damage spells that mimic or have the same effect as diseases?
Would this explanation fit game lore?