get_random_realm_epidemic

get_random_realm_epidemic = {
	random_sub_realm_county = {
		limit = {
			any_county_province_epidemic = { outbreak_intensity >= $INTENSITY$ }
		}
		weight = {
			base = 1
			modifier = {
				add = 750
				has_province_with_epidemic = { intensity >= apocalyptic }
			}
			modifier = {
				add = 5
				has_province_with_epidemic = { intensity >= major }
			}
		}
		random_county_province = {
			limit = {
				any_province_epidemic = {
					outbreak_intensity >= $INTENSITY$
				}
			}
			weight = {
				base = 1
				modifier = {
					add = 750
					any_province_epidemic = {
						outbreak_intensity >= apocalyptic
					}
				}
				modifier = {
					add = 5
					any_province_epidemic = {
						outbreak_intensity >= major
					}
				}
			}
			random_province_epidemic = {
				limit = {
					outbreak_intensity >= $INTENSITY$
				}
				weight = {
					base = 1
					modifier = {
						add = 750
						outbreak_intensity >= apocalyptic
					}
					modifier = {
						add = 5
						outbreak_intensity >= major
					}
				}
				save_scope_as = epidemic_scope
			}
		}
	}
}

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