Can't Touch This

Needs Legends of the Dead, and it's a rated very-hard for good reason: you need a barony that is simultaneously infected AND sitting in a county at 100 plague resistance. The wiki route: Mahasamanta Parakkirama of Pandya, 1066. Build temple holdings in Madurai and upgrade everything; put the Burial Site duchy building in your capital, plus Hospices and Monasteries in every Madurai barony. When a plague reaches the realm, take the Isolate Capital decision and set your best Personal Physician to Control Plagues - the stacked sources push the county to 100 while the infection is live. NB there are unresolved reports of this not unlocking even when the conditions look met (Paradox has it "In Review") - it does unlock for most, but if you're unlucky, re-trigger with a later plague.