The Blue Shadow Virus was a particularly destructive virus. It caused the Candorian plague, which brought devastation to the galaxy many generations prior to the Clone Wars. Later, both the Confederacy of Independent Systems and the Galactic Empire utilized this virus in plots involving biological weaponry.
Generations before the Clone Wars, the virus spread rapidly throughout the galaxy. Fortunately, the Galactic Republic managed to eradicate it. They implemented strict quarantines, confining the Blue Shadow Virus to the affected planets. This prevented further spread, and the virus eventually died out. The public was greatly relieved.
However, a mutated form of the virus persisted, eventually becoming known as the Candorian plague. This plague wiped out the entire population of Candoria and its colonies in 46 BBY.
During the Clone Wars, Dr. Nuvo Vindi, a scientist with an unstable mind, recreated the Blue Shadow Virus. He was aided by Zeta Magnus, a mad genetic terrorist. After capturing Jar Jar Binks and Padmé Amidala, Vindi proclaimed that the virus had not been eradicated, but rather murdered. He also stated that he had recreated life, specifically the life of the virus. While the original virus spread through water and caused immediate death in certain species, Vindi succeeded in creating an airborne version. The airborne version had a longer incubation period compared to the waterborne versions, and victims experienced respiratory symptoms before succumbing to death.
Jar Jar and Padmé attempted to warn Vindi about the dangers of releasing the virus, but he ignored them and proceeded with his plan. However, a unit from the 501st Legion arrived, and the laboratory was successfully secured. Despite this, Vindi's servant droid, LEP-86C8, managed to detonate a remaining bomb, infecting Padmé, Ahsoka Tano, Rex, and numerous clone troopers. Before the virus could kill everyone trapped inside, Anakin Skywalker and Obi-Wan Kenobi discovered the cure: the Reeksa root. After acquiring a sufficient amount of the root from Iego, all the infected individuals were cured.
During the Galactic Civil War, the Galactic Empire employed the airborne version of the virus as a bioweapon, notably during the Dentaal Uprising.
The Blue Shadow Virus was conceived in 2009 and made its debut in the 17th episode of The Clone Wars TV series. In the same year, The Essential Atlas, a reference book, retconed the Blue Shadow Virus, identifying it as the cause of the Candorian plague. The Candorian plague was a disease first introduced in the 1995 magazine Star Wars Adventure Journal 5 and further developed over the following two decades. This retcon was kept after the 2014 canon reboot, as the RPG sourcebook Unlimited Power explicitly linked the Blue Shadow Virus and the Candorian Scourge.