Ochiba - Falling Leaves

Protagonist Kai is currently 17 and starting his second day at Yamada High, on his previous school at age 15 he nearly died due to acute lung failure even though it was unknown beforehand that there was anything wrong at all.

After numerous visits to the hospital, countless X-ray's and neverending CT-scan's, it turned out that one of his lungs was defunct or 'necrotic' since birth, a rare condition that was overlooked during the pregnancy echoes, nobody knew a thing until he collapsed right there on the basketball court.

This led to him having to re-do his last year of junior high, and Rien Tanada his former love interest breaking up with him because they would not be in the same school anymore.