What is the default Plague Frequency game rule in CK3?

Normal: Plagues will appear and spread at a normal rate. (patch 1.20.1.2)

Does changing the Plague Frequency rule disable achievements?

No: none of its 7 settings carries the flag blocks_achievements in patch 1.20.1.2.

Game rule · Difficulty · Health

Plague Frequency

Normal · default
Often
Very Often
Doomsday
Rare
Very Rare
Disabled

Plague Frequency is a game rule in Crusader Kings III (Difficulty and Health) with 7 settings: Normal (the default), Often, Very Often, Doomsday, Rare, Very Rare and Disabled. None of its settings disables achievements. Patch 1.20.1.2, from game/common/game_rules/00_game_rules.txt.

This section as JSON: /raw/game-rules.json

Quick answers (2)

What is the default Plague Frequency game rule in CK3?: Normal: Plagues will appear and spread at a normal rate. (patch 1.20.1.2)
Does changing the Plague Frequency rule disable achievements?: No: none of its 7 settings carries the flag blocks_achievements in patch 1.20.1.2.

7 settings

Normal
Default
Plagues will appear and spread at a normal rate.
epidemic_frequency_normal
Often
Plagues will appear and spread more frequently.
The script checks it 7 times, in 6 places
Script values 6
- outbreak_chance_apocalyptic_mult_value
- outbreak_chance_black_plague_mult_value
- outbreak_chance_dysentery_mult_value
- outbreak_chance_major_mult_value
- outbreak_chance_minor_mult_value
- spread_chance_epidemics_mult_value
Every has_game_rule = epidemic_frequency_often in game/common and game/events, by the top-level object it sits in.
epidemic_frequency_often
Very Often
Plagues will appear and spread much more frequently.
The script checks it 10 times, in 7 places
Script values 7
- outbreak_chance_apocalyptic_mult_value
- outbreak_chance_black_plague_mult_value
- outbreak_chance_dysentery_mult_value
- outbreak_chance_major_mult_value
- outbreak_chance_minor_mult_value
- recent_epidemics_outbreak_mult_value
- spread_chance_epidemics_mult_value
Every has_game_rule = epidemic_frequency_very_often in game/common and game/events, by the top-level object it sits in.
epidemic_frequency_very_often
Doomsday
The air is thick with miasma, and the rivers run rife with corruption. Pestilence is the very air you breathe and the water you drink. There is no respite from the eternal plague.
The script checks it 10 times, in 7 places
Script values 7
- outbreak_chance_apocalyptic_mult_value
- outbreak_chance_black_plague_mult_value
- outbreak_chance_dysentery_mult_value
- outbreak_chance_major_mult_value
- outbreak_chance_minor_mult_value
- recent_epidemics_outbreak_mult_value
- spread_chance_epidemics_mult_value
Every has_game_rule = epidemic_frequency_doomsday in game/common and game/events, by the top-level object it sits in.
epidemic_frequency_doomsday
Rare
Plagues will rarely appear and spread with less frequency.
The script checks it 6 times, in 6 places
Script values 6
- outbreak_chance_apocalyptic_mult_value
- outbreak_chance_black_plague_mult_value
- outbreak_chance_dysentery_mult_value
- outbreak_chance_major_mult_value
- outbreak_chance_minor_mult_value
- spread_chance_epidemics_mult_value
Every has_game_rule = epidemic_frequency_rare in game/common and game/events, by the top-level object it sits in.
epidemic_frequency_rare
Very Rare
Plagues will very rarely appear and spread with much less frequency.
The script checks it 6 times, in 6 places
Script values 6
- outbreak_chance_apocalyptic_mult_value
- outbreak_chance_black_plague_mult_value
- outbreak_chance_dysentery_mult_value
- outbreak_chance_major_mult_value
- outbreak_chance_minor_mult_value
- spread_chance_epidemics_mult_value
Every has_game_rule = epidemic_frequency_very_rare in game/common and game/events, by the top-level object it sits in.
epidemic_frequency_very_rare
Disabled
Plagues will not appear.
The script checks it 24 times, in 11 places
Epidemics 7
- bubonic_plague
- consumption
- dysentery
- ergotism
- measles
- smallpox
- typhus
Events 3
Script values 1
- outbreak_chance_black_plague_default_value
Every has_game_rule = epidemic_frequency_disabled in game/common and game/events, by the top-level object it sits in.
epidemic_frequency_disabled

Source: game/common/game_rules/00_game_rules.txt line 249 (key other_epidemic_frequency), modifiers from game/common/modifiers, names from the English localisation; Crusader Kings III 1.20.1.2.