Between Bloom and Breath (A Shin Soukoku Hanahaki AU)

In this AU, Hanahaki Disease is rare and poorly understood. When someone falls deeply in love but believes their feelings can never be returned, flowers begin to grow inside their lungs. At first, it's only an occasional cough and a few stray petals. Eventually, the flowers spread, making it harder and harder to breathe.

Akutagawa is the last person anyone would expect to develop Hanahaki.

He dismisses the first petals as a weakness. The second time, he burns them. By the third time, he realizes exactly what they mean.

The problem is that the person he loves is Atsushi.

Akutagawa refuses treatment. He refuses to tell anyone. Most importantly, he refuses to tell Atsushi.

Meanwhile, Atsushi notices something is wrong. Akutagawa is missing missions, covering his mouth when he coughs, and growing more exhausted every day. Their rivalry has softened over the years, and despite Akutagawa's constant attempts to push him away, Atsushi can't ignore the fact that his partner is suffering.

Convinced that Akutagawa is hiding an illness or injury, Atsushi begins investigating.

The twist is that Hanahaki isn't caused by unrequited love in this universe.

It's caused by believing your love is unrequited.

And Akutagawa is wrong.

Atsushi has been in love with him for months.

Neither of them realizes the other feels the same way. Atsushi assumes Akutagawa dislikes him. Akutagawa assumes Atsushi could never see him as anything more than a former rival.

As Akutagawa's condition worsens, Atsushi becomes increasingly desperate to find out what's happening. The story revolves around misunderstandings, awkward attempts at caring for each other, and both of them slowly learning to communicate.