It's not actually a curse it's an STD or a very viral disease she has but since she's the saint her symptoms are cured but she is still affected but since the people she has had relationship with don't regenerate they end up with the full brunt of it. Think of her as asymptomatic but still viral essentially everyone on that village is suffering from an epidemic
I think that the saint has to understand what the disease is in the first place. Her power allows her to cure things she understands. But if they believe its some curse and not a disease she can't treat it. Im not 100% as it hasn't been revealed yet but I believe her powers work like that or atleast that would make the most sense

Mhm can someone explain chapter 41; I didn’t understand anything lol