Toli-X was a virus, a mutated strain, that propagated across different planets. Its transmission occurred via asteroid mold, leading to the contamination of numerous worlds around 54 BBY. While it proved fatal to Humans without medical intervention, a vaccine was rapidly developed after its emergence, offering a cure for the Toli-X infection. In 44 BBY, during a mission to Kegan, Jedi Padawans Siri Tachi and Obi-Wan Kenobi uncovered that this Outer Rim planet had experienced a significant Toli-X outbreak seven years earlier. However, due to their isolationist government, they remained unaware of the vaccine's existence. The parents of V-Davi, a local boy whom Tachi and Kenobi encountered, had died from the virus when V-Davi was just two years old. Initially, to spare him further sorrow, the Jedi apprentices chose to withhold the information about the Toli-X cure. They eventually revealed it as a crucial element to free V-Davi from the brainwashing inflicted by the Re-Learning Circle.
The virus known as Toli-X was initially introduced in The Fight for Truth. This novel, released in 2000, marked the ninth book in the Jedi Apprentice series, a collection of young adult novels written by Jude Watson.