Game concept
Plague
The game also links it as Plagues.
From the game's encyclopedia
A Plague is an outbreak of a disease that spreads across the map, causing both local effects and infecting characters in its area. Affected Baronies have an Infection Rate that handles their severity and can be countered by improving the Plague Resistance.
A Plague can have different Plague Intensities, affecting its area of effect, duration and Fatalities.
The game's text for patch 1.20.1.2. Words in blue are entries of the encyclopedia: hover one to read it, as in the game.
Plague is an entry of Crusader Kings III's in-game encyclopedia. The game explains it: "A Plague is an outbreak of a disease that spreads across the map, causing both local effects and infecting characters in its area." It covers Infection Rate, Plague Resistance, Chance of Infection and Plague Intensity. The game also links it as Plagues. Patch 1.20.1.2, from game/common/game_concepts/06_ce1_game_concepts.txt and game/localization/english/game_concepts_l_english.yml.
Source: game/common/game_concepts/06_ce1_game_concepts.txt, game/localization/english/game_concepts_l_english.yml and the English localisation, Crusader Kings III 1.20.1.2.