Yuri has the Hanahaki disease. It's a disease in which she coughs up flower petals that grow in her lungs because Yuri suffers from one-sided love. If not treated, the disease is fatal. It can be cured when Yuri's love interest returns his feelings for the rest of their lifes. But her love interest didn't love her back.

Bakugo noticed Yuri coughing a lot during class. She stood up, asking to go to the nurse. Her coughing had gotten a lot worse the last few days.