The plague is a virus similar to cerebrospinal meningitis attacking the brain. It initially causes weakness and a high fever, often leading the host to become disoriented or possibly collapse. The virus can cause the kidneys to shut down and eventually kills the afflicted. Earth drugs (and presumably Antibiotics) have no effect on the virus. (SG1: "Frozen")
The exact origin of the plague lies with the Ancient Amara who created the plague under the instruction of the Doci of the Ori in an attempt to convert the Ancients in the Milky Way to Origin. The plague was unleashed first on two planets, Vis Uban and Pedion Elysium and then decimated the other Ancient colonies (also most likely affecting other species). To avert the plague the Ancients created a time machine to go back and rewrite their history, however the machine never worked properly just creating a bubble in space that looped time every few hours. The Ancients eventually shut off the machine and let the end come. (SG1: "Window of Opportunity", "Fallen", "Transitions")
As a means of escaping the plague the remaining Ancients either attempted to achieve Ascension while others traveled to the Pegasus galaxy, in the City-ship Atlantis. One Ancient Ayiana was left behind for an unknown reason (most likely because she was a plague carrier). (SGA: "Rising")
The Ancient Ayiana was a carrier of the plague and inadvertently infected SG-1 and some scientists at a research outpost in Antarctica. The infected were also healed by Ayiana. It is also apparent that symbiotes are able to heal this affliction. When the Priors began converting the Milky Way, one of their ploys was to release a disease similar to this plague, and then heal the infected. (SG1: "Frozen", "Abyss", "The Powers That Be")
Daniel Jackson would note the similarity of the Ori plague to the Ancient contagion.