What sickness does Thena have in Eternals?

In the Marvel film Eternals, what illness does the character Thena suffer from? Does this sickness affect her powers or her ability to fight alongside the other Eternals? Does the film provide any insight into the cause or origin of this sickness, and how it impacts her character development throughout the story? As a viewer, do you find Thena's struggle with this illness adds depth to her character or serves a specific purpose within the narrative?