Possible Clues:

• Communicators briefly suffering setback during Great Plague
• Single brief medical upset at centre of large-scale outbreak
• Widespread infection of the outer skin? That's not right
• Heroic adventure about the same eruption
• Last couple escapes death in Great Plague
• Plague half contained in saga
• Disease affecting many in a population at the same time
• Citrus fruit
• Not localized