The Black Death appears to hit almost all cities. Sit tight if in a RAT city. If not, move out fast, it is coming your way. I found if stays around for 4-6 turns most of the time if I do nothing. Jest one unit recruited or repaired appears to resets the timer for that city jest like moving in or out. You should of gotten a heads up first, something about the Great Mortality moving up the trade routes? I sit for one complete turn after ALL the RATs leave a city before I do a condense and start rebuilding. Normal Plague hits only a few cities at a time unlike the Great Death. I do have to admit I was rolling in cash 2 turns after it left. SadCat