[This is my own idea of a disease :D]

You heard about a Japanese myth about the "Ōkabamadara disease" Monokuma said as the motive. You personally thought it was insane and left the area to go find Nagito.

You walked into his cottage, opened the door which was surprisingly unlocked and walked in. Before you could say anything you saw a flower pattern on his hand. It was the pattern of the Spider Lily flower. Just like in the Japanese myth Monokuma told you, and there was a butterfly on his other hand, which was also mentioned by Monokuma that spread the disease of this motive...