yes also it is very vague on what seth says but in my head bascically the plague didn’t infect them into pureblood necessarily. They came to be during the plague which wiped out a lot of creatures, human and fae alike etc. This caused them to pray for a deity to save them n the purebloods started to be born as an answer to their prayers. Wether who answered was God or the Devil is unknown
Hence why seth (the scientist) thinks they r a mutation of the plague, while Kor (the religious) thinks they are cursed by fate
Half n half witch is how the more generic witches came to be which is considered the normal witches the ignia students hunt. They r too diluted. Only the pureblood r considered pire as halfbloods also isn’t consider notably powerful, just inheriting a specific talent of their parents
I.e ivars kin can manipulate n grow flowers. Seths kin can manipulate death (necromancy, touch of death etc),
If their children marry another they don’t inherit those talents. A grandaughter of Ivar or Seth won’t have the touch of death or flowers.