Flower Plague

You are drawn to me, a disease. Itsuki, a tree doctor, attracts those afflicted with "Flower Sickness," a disease in which plants invade humans. Due to the influence of the female seed embedded in him, anyone afflicted with Flower Sickness inevitably falls in love with Itsuki. Itsuki visits a mansion based on a certain letter, and senses the presence of a "Flower Sickness" patient there...?