The Reason Behind The Dropout

What if there was a different reason for Saint Snow dropping out of the running for Love Live? a much darker reason than just messing up on stage. What if one of them became very sick and they physically couldn't perform? How would this affect them? How could this affect their friends? Who would be physically affected by this illness?

I had this idea for a few days now. Now I'm making it a real story

And the cover of this story probs gives away who it affects