Hokenshitsu no Shinigami is a manga series created by Shou Aimoto, serialized in Weekly Shōnen Jump from 2009 to 2011. It follows school nurse Hadesu Itsuhito, who is powerful enough to get rid "disease demons" from possessing students' bodies.
Chocolate Complex (Volume 1, Chapter 21)
A special kind of chocolate is popular with students. However, it contains a demon disease that causes them to swell up.