Deezer was a low-level scientist on Jora.
The 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.
Deezer spent two years developing an antibody to the Syntax. When he met the Eighth Doctor and Izzy Sinclair, he was suspicious at first, but then welcomed them. However, the Doctor found out that Deezer was trying to kill the Syntax and sabotaged his antibody. Defeated, Deezer accepts the Syntax' influence and joins the rest of his people. (PROSE: Syntax)