Game concept
Infection Rate
The game also links it as Infection Rates.
From the game's encyclopedia
Baronies affected by Plagues have an Infection Rate that handles their severity and the strength of its effects.
The Infection Rate will always progress towards its maximum value if nothing is done to fight it. Minor, Major and Apocalyptic Plagues will progress at different velocities, marked by their intensity.
In order to mitigate the impact of the Infection, Rulers can improve the Plague Resistance of their Realm.
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.
Infection Rate is an entry of Crusader Kings III's in-game encyclopedia. The game explains it: "Baronies affected by Plagues have an Infection Rate that handles their severity and the strength of its effects." It is part of Plague. The game also links it as Infection Rates. 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.