{Person a} loves {person b} but these feelings are one sided so they end up developing hanahaki Disease. Due to this they end up suffering the effects of the disease and {person b} notices.

{Hanahaki Disease}

This disease happens when someone has one sided feelings for someone. They cough up/throw up flower petals{flowers} that are either their favorite flower type or favorite color. This can be removed by surgery but the person's romamyic feelings towards them will be gone. They could also, lose their memories of the person they love or their ability to ever love again. Another way to cure this is if the person they love returns their feelings. If neither happens then their lungs will fill up with flowers causing them to suffocate and die.