An unnamed male humanoid, "Patient Zero" was born sometime during the Golden Age of the Sith. Karness Muur, one of the first Dark Lords of the Sith, used Sith alchemy to engineer the Rakghoul plague, a highly contagious infection intended to be used to create an army of mindless beasts to rule the galaxy. This unnamed humanoid became the first person to be infected by Muur's creation, by having the Sith magic from the Muur Talisman used on him.
Thousands of years later, Darth Acina, a newly made Dark Council member, held him in stasis in the Arcanum, a space station used to hold dangerous Sith artifacts. However, Lord Tagriss raided the station by order of the Dread Masters and set Patient Zero free.
Patient Zero is a level 53 champion enemy NPC encountered in Star Wars: The Old Republic, added in patch 2.0.0. He appears during the seeker droid questline and is classified as "Rakghoul Progenitor."