{this is my attempt at a headcannon. Sorry if it is terrible}

Octavian (for some reason) wanted to be friends with Nico. He went to Percy for help on how to connect with him. Percy hated Octavian (duh) so he said that Nico hated his actual sister Bianca. Percy told him that if he made fun of her in front of him, he might want to be friends.

Octavian took the advice seriously and went up to Nico, bad mouthing Bianca, while Percy hid in bushes and watched, trying not to laugh.

Let's just say Octavian stayed in the infirmary for two months.

Nico eventually found out that Percy told Octavian to do that.

Percy was also in the infirmary for one month.