According to supplementary material,
X as we know it was originally not going to be a part of the
Final Fantasy series at all, but was planned as a separate game called
Eighteen. Its plot would have centered around a mysterious disease that kills people on their 18th birthday, and would have had Yuna cast as a doctor/nurse who traveled around the world administering a cure to those who were found to be infected. It would later be revealed that this alleged "cure" was actually the
cause of the disease, with the organization Yuna worked for using the "disease" as a means of controlling the world by alleging that they were the only ones capable of providing the cure while simultaneously perpetuating the disease's existence. When the game was later redesigned to be the next
Final Fantasy installment, several of these plot points were retooled to fit the new game's story.