The Blue Shadow Virus was a deadly disease that spread like a plague in the galaxy many years before the Clone Wars. Waterborne in its original form, the majority of carbon-based lifeforms were susceptible to it. Although the virus was eventually eradicated, during the Clone Wars, Doctor Nuvo Vindi of the Confederacy of Independent Systems re-engineered an airborne strain of the virus on Naboo. He attempted to spread it across the planet, but was stopped by the Galactic Republic and Jedi Order.
The waterborne virus in a Naboo river
The Blue Shadow Virus, in its natural form, was a waterborne virus. A victim infected with the virus only had 48 hours to live before it became fatal. It was notorious for being supposedly incurable, with a 96% success rate in killing carbon-based life-forms. An antidote was able to be synthesized with reeksa root, a rare plant only found on the planet Iego. Beings infected with the disease developed visible blue veins, and in some cases the rest of an infected individual's skin would also turn extremely pale. Other symptoms included coughing and fatigue.
The Blue Shadow Virus in gaseous form
Several thousand years before the Clone Wars, the Blue Shadow Virus encompassed the galaxy, killing millions who drank contaminated water. Efforts to eradicate the virus succeeded and it was erased from the galaxy, only to be reintroduced by Doctor Nuvo Vindi during the Clone Wars as part of the research effort of Jedi-proof biological weapons. Vindi created several bombs that held an airborne version of the virus, which he planned to release on several key systems that were involved in creating galaxy-wide war. Though the release of Vindi's bombs across the galaxy was stopped by the Jedi, some of the virus was released inside of Vindi's laboratory on the planet Naboo. Senator Padmé Amidala, Jedi Padawan Ahsoka Tano, and several clone troopers were trapped inside the lab and infected, but fortunately, the disease itself did not escape. Jedi Master Obi-Wan Kenobi, Jedi Knight Anakin Skywalker, and Captain Gregar Typho of the Royal Naboo Security Forces found out about the existence of reeksa root from the planet Iego, which could be used to create an antidote. After the Jedi obtained the root from Iego, all who had been infected who were still alive were cured.
During a rebellion against the Galactic Empire conducted by the Rebel Alliance decades later, the virus was stated to have been weaponized multiple times in past wars. Major Capin Harinar of Rebel Alliance Intelligence Service wrote an analysis to General Davits Draven about Imperial weapons projects. The analysis detailed the Electro-Proton Bomb/Defoliator, the mega-ion cannon, and the Blue Shadow Virus. In regards to the virus, Harinar stated that the Alliance would be severely ill-prepared to guard against such a targeted pandemic. The analysis was part of a larger report from Draven to Commander-in-Chief Mon Mothma, summarizing Harinar's report and Colonel Haxen Delto's report.
At one point, schematics for harnessing the virus' deadly power were leaked. Though most factions condemned the use of the virus, some vicious groups chose to produce a strain of the virus in the form of grenades.