User blog comment:B214/Question and Answers 2/@comment-4547267-20140420141553/@comment-16281343-20140420143511

Saji caught the Dragon Flu, the same flu that Issei caught in Volume 13, Life 4. Issei then helped out the Student Council in place of Saji.

I don't know any specific things other than that since that SS is haven't read it yet and still on my to-read list.

P.S. : I think this is the SS where Issei first knew who the girls of the Student Council like.