What is the default Minor Diseases game rule in CK3?

Default: Minor diseases will appear as per normal. (patch 1.20.1.2)

Does changing the Minor Diseases rule disable achievements?

Yes for “Fewer”: the file gives it the flag blocks_achievements. The other settings keep achievements on.

Game rule · Difficulty · Health

Minor Diseases

Default
Fewer ✕

Minor Diseases is a game rule in Crusader Kings III (Difficulty and Health) with 2 settings: Default (the default) and Fewer. “Fewer” 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 Minor Diseases game rule in CK3?: Default: Minor diseases will appear as per normal. (patch 1.20.1.2)
Does changing the Minor Diseases rule disable achievements?: Yes for “Fewer”: the file gives it the flag blocks_achievements. The other settings keep achievements on.

2 settings

Default
Default
Minor diseases will appear as per normal.
default_minor_disease_frequency
Fewer
Disables achievements
Minor diseases will appear less often.
The script checks it 2 times, in 2 places
Events 1
- … A Journey's End?
Script values 1
- disease_outbreak_pulse_chance_of_no_event
Every has_game_rule = fewer_minor_disease_frequency in game/common and game/events, by the top-level object it sits in.
fewer_minor_disease_frequency

Source: game/common/game_rules/00_game_rules.txt line 265 (key minor_disease_frequency), modifiers from game/common/modifiers, names from the English localisation; Crusader Kings III 1.20.1.2.