FG36 was a nanovirus of the Separatist faction, engineered during the Clone Wars by the scientist Ovolot Qail Uthan. Its purpose was to kill the clone troopers of the Republic, who were soldiers derived from the genetic material of the bounty hunter Jango Fett.
The Republic clone commando unit known as Omega Squad captured Uthan on the planet Qiilura before she could finalize the virus. It had not yet been specifically tailored to the unique accelerated aging trait of the clones. Following her capture, Chancellor Palpatine managed to obtain several samples of the virus.
Subsequently, in 19 BBY, after the Galactic Empire had risen to power, Uthan's homeworld of Gibad refused to submit to the Empire's demands. As a result, Palpatine gave the order to release the FG36 samples he had been storing onto the planet, resulting in the death of all exposed inhabitants. Later on, Uthan developed an antigen for the virus and, with assistance from the Mandalorian Kal Skirata and his boys, disseminated it across Mandalore, thereby immunizing the entire planet against the virus.
The nanovirus FG36 made its initial appearance, although without a specific designation, in the 2004 novel titled Republic Commando: Hard Contact, authored by Karen Traviss. The nanovirus was later given its official identification in the 2009 novel Imperial Commando: 501st, which was also penned by Traviss.