Joran

The Jorans were natives of the planet Jora.

At some point in their history, Joran scientists tried to develop the ability to control pheromones. This led to the idea of a pheromone language. However, the language became a living, sentient thing, known as the Syntax, that spread to the entire population, save Deezer. It subjugated the will of each Joran, leaving them unable to make their own decisions.

The Eighth Doctor reasoned that, as the Jorans were happy in their new condition, he would not allow Deezer to kill the Syntax. (PROSE: Syntax)