add_plague_county_modifiers

add_plague_county_modifiers = {
	if = {
		limit = {
			any_sub_realm_county = {
				development_level >= 30
			}
		}
		show_as_tooltip = { # Show as tooltip to reduce tooltip size
			every_sub_realm_county = {
				custom = shattered_by_plague_tt
				add_county_modifier = {
					modifier = shattered_by_plague
					years = 10
				}
			}
		}
	}
	if = {
		limit = {
			any_sub_realm_county = {
				AND = {
					development_level >= 20
					development_level < 30
				}
			}
		}
		show_as_tooltip = { # Show as tooltip to reduce tooltip size
			every_sub_realm_county = {
				custom = ravaged_by_plague_tt
				add_county_modifier = {
					modifier = ravaged_by_plague
					years = 10
				}
			}
		}
	}
	if = {
		limit = {
			any_sub_realm_county = {
				AND = {
					development_level >= 15
					development_level < 20
				}
			}
		}
		show_as_tooltip = { # Show as tooltip to reduce tooltip size
			every_sub_realm_county = {
				custom = blighted_by_plague_tt
				add_county_modifier = {
					modifier = blighted_by_plague
					years = 10
				}
			}
		}
	}
	if = {
		limit = {
			any_sub_realm_county = {
				AND = {
					development_level >= 10
					development_level < 15
				}
			}
		}
		show_as_tooltip = { # Show as tooltip to reduce tooltip size
			every_sub_realm_county = {
				custom = struck_by_plague_tt
				add_county_modifier = {
					modifier = struck_by_plague
					years = 10
				}
			}
		}
	}
	hidden_effect = {
		every_sub_realm_county = {
			if = { # Refresh the modifier for new plagues
				limit = {
					has_county_modifier = struck_by_plague
				}
				remove_county_modifier = struck_by_plague
			}
			else_if = { 
				limit = {
					has_county_modifier = blighted_by_plague
				}
				remove_county_modifier = blighted_by_plague
			}
			else_if = { 
				limit = {
					has_county_modifier = ravaged_by_plague
				}
				remove_county_modifier = ravaged_by_plague
			}
			else_if = {
				limit = {
					has_county_modifier = shattered_by_plague
				}
				remove_county_modifier = shattered_by_plague
			}
		}
		every_sub_realm_county = {
			limit = {
				development_level >= 30
			}
			add_county_modifier = {
				modifier = shattered_by_plague
				years = 10
			}
		}
		every_sub_realm_county = {
			limit = {
				AND = {
					development_level >= 20
					development_level < 30
				}
			}
			add_county_modifier = {
				modifier = ravaged_by_plague
				years = 10

As written in game/common/scripted_effects/06_dlc_ce1_epidemics_effects.txt.