Talk:Kishou Arima/@comment-74.12.101.156-20160406212930/@comment-10733658-20160406213137

It's explained in the opening of the article.

"However, there are many who are envious of Arima's combat power, and older investigators who realized their own great inferiority to Arima often suffer from "Arima disease"."

It's basically when older Investigators like Marude are jealous of Arima for making them look bad.