An ailment that afflicted the Ancients (Alterans) living in the Milky Way Galaxy many millions of years ago. The plague wiped out whole colonies of Ancients living on planets throughout the galaxy, and is believed to be their primary motivation for seeking ascension. By shedding their physical bodies and living on another plane of existence as beings of pure energy, the Ancients escaped the extinction of their civilization.
One colony of the advanced species even developed time manipulation technology, using a localized network of Stargates to create a bubble in which time repeated over and over again. Their hope was either to go back and change the past, or buy themselves enough time to find a cure for the disease from which they were suffering. But eventually they gave up, turned off the device and accepted their fate.
The plague is also the most likely reason for the Ancients' decision to move to the Pegasus Galaxy, flying the city ship Atlantis from Earth (where it once sat on the Antarctic continent) to Lantea several million years ago. Some of their people, including the woman Ayiana, were left behind on Earth -- probably because they had been infected with the disease.
The disease did not affect all Ancients the same, with some of them acting as carriers but able to stave off the effects of the disease with their own remarkable healing powers (as in the case of Ayiana). Having been seeded by the Ancients, human beings are also evolutionarily close enough to them to be susceptible to the disease, as well.
The exact nature and origin of the plague remain unknown, including whether it was a natural occurance or a biological attack by an enemy.
Window of Opportunity - SG-1 finds a civilization devastated by a plague, with a time-loop device that its creators ultimately failed to perfect.
Frozen - A team in Antarctica finds an evolutionarily advanced human, Ayiana, frozen in the ice, and revives her -- only to catch the disease she is carrying.
Rising, Part 1 - Several million years ago Atlantis departs from Earth, leaving Ayiana behind.
Threads - Anubis reveals that the Ancients engineered the super-weapon on Dakara to re-seed life throughout the galaxy in the wake of the plague's devastation.