The Plague

[Megas Alexandros]

You say the infected NPC spies roamed around for 30 years. Did they not die from the disease themselves?

[professorspatula]

Agents can have the plague for years without dying. Sometimes it kills them in a few turns, other times they're almost immune to its ill effects. I've had mine infect AI settlements for over 20 years at a time. One of the enemy spies might have died from it, but the other one I couldn't get rid of for ages.