The Syntax was a living, sentient language.
The Syntax was created by Joran scientists as they tried to make a pheromone language. However, the Syntax evolved into a living language which then spread throughout the population of Jora. It subjugated the Jorans' wills, leaving them without individual identities.
The only unaffected Joran, Deezer, created an antibody to kill the Syntax. However, the Eighth Doctor altered the chemical agent, making it harmless to the Syntax. (PROSE: Syntax)