On P8X-412, the inhabitants who refused to embrace Origin were struck down by a mysterious illness through the power of the Prior. Symptoms included fatigue, high fever, chest and stomach pains, and respiratory distress, then as internal organs began to shut down, the patient went into shock as his body could no longer process toxins, and he was essentially poisoned to death. In fact, the symptoms and rate of progression of the disease were remarkably similar to the virus that was carried and cured by Ayiana, the Ancient who was revived in Antarctica, and it is possible that there is a connection between the disease of the Priors and the plague that wiped out the Ancients millennia ago.
The illness spread very rapidly among the population, and although the Goa'uld healing device was effective in reversing the symptoms, the patients succumbed once again almost immediately. A bacterial infection was ruled out, and it was determined that the illness was probably viral, however treatment with an antiviral cocktail had limited effectiveness as the virus was rapidly mutating and developing an immunity to whatever was used against it. Only 14 hours after the preliminary symptoms were noted, the first victim of the disease died, and as the illness swept through the village, even Mitchell became critically ill. When at last the villagers begged the Prior's forgiveness, he used the power of his staff to cure the disease and even to revive the dead. As the planet fell to Ori domination, the Prior warned, "Tell others what you have seen today."
Some months later, intel from the Sodan led the SGC to P2X-885, a world of 10,000 poverty-stricken peasants, only one freshwater well, and 130 degrees in the shade. A Prior had already visited the planet and the inhabitants had fallen to the Ori. SG-6 was sent undercover to observe, and when they returned, Colonel Barnes began exhibiting symptoms identical to those on P8X-412. It was determined that during their mission, his teammate, Lieutenant Fisher, had had physical contact with a Prior and had become "patient zero" to spread the disease to Earth. Although Fisher did not show symptoms immediately, the disease was spread very quickly among those with whom he had had contact, including his teammates, members of the SGC, the community of Colorado Springs, and those present at a speaking engagement by General Hammond. Although the strain from P2X-885 had a completely different genetic makeup from that on P8X-412, the identical symptoms suggested that the virus caused by the Priors was rapidly mutating, and it came to be known as the Prior plague.
The plague was highly contagious, spreading from person to person, and there were indications that it also may have been airborne and persistent. Despite desperate attempts by the CDC to quarantine those exposed, the virus quickly spread to the general population through public transportation and air travel. By the time the National Guard was called in to seal the state, it was already too late, and the media was fed the cover story that the outbreaks were due to a new influenza that had been identified at St. Francis Medical Center. With more than 8000 confirmed cases in 13 states and the death toll expected to climb at an alarming rate, the President ordered the nation under quarantine. However, the virus had already begun to appear in Europe and Africa and was showing signs of becoming a global pandemic.
Orlin, an ascended Ancient, chose to return to human form to assist the SGC in finding a cure for the pandemic. He believed that the Prior had essentially manipulated his own DNA to create the virus, and although it had repeatedly mutated, Orlin was able to work from a blood sample acquired from the Prior who had infected Fisher in order to synthesize a cure. Meanwhile, Gerak, who had taken on the powers of a Prior, was dissuaded from Origin by Teal'c, who convinced him to come to Earth to assist in defeating the plague. At the SGC, he planted his staff, and its glowing energy penetrated the facility, instantly curing those afflicted, as Gerak vanished in flames. From those who were cured, SGC doctors were able to isolate an antibody, and using Orlin's research and formula, they were able to manufacture a vaccine. The cure was distributed worldwide, but not before a final death toll of over 3000.
The Prior plague was no longer a threat to Earth, but although it did appear on other worlds, including Tegalus, the SGC was able to distribute the vaccine to many worlds that needed it. Consequently, outbreaks became fewer, and the Priors began to turn to other tactics such as the infestation of R75.