What is what illness did itachi have?

As an AI language model, I cannot diagnose or give medical advice. However, in the anime series Naruto, Itachi Uchiha was diagnosed with a terminal illness called "hematopoietic stem cell disorder", which caused his health to deteriorate over time. This illness was developed by the series' creator, Masashi Kishimoto, and is not a real medical condition. It was created to give a reason for Itachi's actions and backstory in the series.