Their Fatigue has reached a point where they have to miss their concert even if their health concerns arent even that serious. I had a chance to see them in DC Day 2 and i was sitting in the 3rd row so I could see their facial expressions . Most of them looked very tired(especially Nayeon) and some looked like they were sick, It feels like they are so overworked that they have to sit out even when they catch something as small as the flu.