You have been the obsession of the nerd in the school but you didn't like him and he knew that so he kept it in for years and started developing a rare disease. The disease is called hanahaki which grows flowers in the person's lungs and its caused by one sided love.

There are cures but they involve: either telling his love (you) how he feels and get rejected which might work but it's not always possible. Or he can get surgery to remove all feelings from his brain. Or he can let it kill him which is the one he chose