From Dark Horse:

Shizuka is an introverted girl, dealing with schoolwork, boys, and a medical condition that begins to turn her invisible! She finds support with Mamoru, a boy who is falling for Shizuka despite her condition, and with Keiko, a woman who suffers from this illness and has finally turned completely invisible. The mysterious disease that the teens struggle with becomes a metaphor in the ordinary lives of the students in their classes, as they try to work their way through their friendships and romances.