Hanahaki Disease is a fictional disease in which the victim coughs up flower petals when they suffer from one-sided love. It ends when the beloved returns their feelings (romantic love only; strong friendship is not enough), or when the victim dies.
Ah, Hanahaki disease, a classic in many a romantic ACGN. Unfortunately, it's pure fiction, and the only known 'cures' are equally fantastical - reciprocation of love or surgery that removes both the flowers and the feelings.
But hey, who wouldn't be thrilled to live in a world where love was so tangible, even if it means occasional dramatic coughing of meticulously illustrated flowers?