The signs had been subtle at first. You'd notice Shadowheart losing focus, or struggling to concentrate. She, of course, brushed it off. "I'm just tired," she'd say. At first, you accepted that excuse. Maybe that recurring pain had been bothering her more recently, making it harder for her to sleep. But it was gradually getting worse.
It went from losing focus to Shadowheart showing signs of confusion. Or taking longer to understand something, requiring instructions or plans to be repeated to her. You were patient with her, and she was grateful for that. But as her condition deteriorated, your worry only increased.
What really triggered concern was how she was acting in battle. In the moments that could make up life or death, Shadowheart couldn't concentrate on her decision making. Her attacks were slow and sloppy, and she nearly stumbled whenever she dodged. Because of this, she made a mistake that could've gotten the party seriously hurt if Lae'zel hadn't stepped in. When the gith'yanki confronted her about it, the defiance usually present in Shadowheart's eyes was quiet, dampened. Something was definitely wrong.
That night at camp, you kept an eye on Shadowheart. She kept to herself as she usually did, but she didn't come to the campfire for food with everyone else. You brought her food over, and she mumbled a 'thank you' absentmindedly. Later when she moved to her bedroll to sleep, you saw that that food was still there, untouched.
The night was relatively peaceful. Until a sound awoke you. It was still dark out, the moon bathing the camp in its light. Looking around, you saw that the source of the noise was Shadowheart, wandering and stumbling around camp. You got up, thinking maybe she was sleep-walking. But as you placed a hand on her shoulder, your hand jerked away in surprise; the half-elf's skin was boiling. She was hot to the touch. And suddenly her behavior made sense; she was sick.