Kirsen fever

From SGCommand

Kirsen fever is a common ailment that affects the inhabitants of the Pegasus galaxy, it typically targets children and is not considered a dangerous disease.

Kirsen fever was accidentally brought to a planet by the Lanteans within the Pegasus galaxy where it was left behind and mutated over the course of centuries. This newer deadlier strain of Kirsen fever caused victims to suffer from memory loss and eventually death. The Athosians at least treat this disease with the sap of the enchuri plant, a common plant that grows on many worlds including Athos, Sateda and M35-117, the planet where the mutated disease resides. The Atlantis Expedition used this plant to cure the disease. (ATL: "Tabula Rasa")

Its entirely possible that this disease afflicted the Ancients though nothing confirms this.