Kai-Kai Bugs

 Kai-Kai bugs are a type of insect that Fea Kreuz Raggs told Teito Klein about, as revealed in a flashback. When Teito was a child and refused to take a bath, Kreuz said that Kai-Kai bugs would crawl into Teito's body. In turn, Teito warned Capella about this when he refused to take a bath.

However, their existence in the series is debatable, as Kreuz could have just been making it all up in order to trick Teito into taking a bath as Frau had never heard of Kai-Kai bugs before. 

Ecology
If they are indeed real, Kai-Kai bugs can be assumed to thrive in dirty or unhygienic conditions.

Trivia

 * http://en.wikipedia.org/wiki/Kai_(name)