They were complaining about needing sick leave and being sick. Now they don't so that but still at 90%. Also several of the workers are wearing cough masks, any way to resolve that?
Is your water in the green? Is there a dead dweller nearby maybe?
Did anyone die in the room before? There's other threads about a bug where sometimes when people die, a bug will occur where the dwellers in that room still think there's a dead person in there until you destroy and rebuild that room.
I figured it out, I saw that Rushes make people happy. I rushed and people got happy.
Another sure way to make them happy is to help them get jiggy wit it. But that's to be expected from a round of hide the cannoli.
Sometimes when they've taken damage it your dwellers will complain about feeling sick. A stimpack usually fixes it for me.