Game concept
Plague Intensity
The game also links it as Plague Intensities, Apocalyptic, Major, Minor.
From the game's encyclopedia
A Plague can be Apocalyptic, Major or Minor, affecting its area of spread, the duration and the Characters that get infected by it, causing 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 Intensity is an entry of Crusader Kings III's in-game encyclopedia. The game explains it: "A Plague can be Apocalyptic, Major or Minor, affecting its area of spread, the duration and the Characters that get infected by it, causing Fatalities." It is part of Plague. The game also links it as Plague Intensities, Apocalyptic, Major and Minor. 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.