Athy is only really suffering because she has a bunch of excess mana. Since she never learned how to use it, it could only build-up and slowly eat at her body. Claude has a curse, however it only affects him because he used black magic on himself (and if you remember, black magic never brings anything good). Lucas poking at her head was to get rid of traces of black magic which were from Jeanette since her entire existence is literally made from black magic. (Which is also why every time Athy interacts with Jeanette, Ijekiel, or "Mr. Whytte" Lucas always comments that she has something dirty on her). As for why Claude and Athy collapsed right before the amnesia incident, it's due to the hair accessory Athy got from Jeanette. The black-haired guy staying at the Alpheus house (can't remember his name atm ) put a spell on it which I believe caused Raven/Blackie to have the urge to return to Athy's body. This caused an abundance of mana to flow into her body, nearly destroy her body, and collapse (as it did when she was younger). Claude collapses because he took/"ate" the excess mana from Athy's body however this sped up the affects of the curse on his body, resulting in the current amnesiac.