bd_random_game_rule_effect

bd_random_game_rule_effect = {
	random = {
		chance = 1
		random = { # You cannot set a chance lower than 1% so we have to do this ugly fix
			chance = {
				value = 1
				if = {
					limit = { years_from_game_start > 100 }
					add = 10
				}
				if = {
					limit = { years_from_game_start > 200 }
					add = 14
				}
				if = {
					limit = { years_from_game_start > 300 }
					add = 15
				}
				if = {
					limit = { years_from_game_start > 400 }
					add = 20
				}
				if = {
					limit = { years_from_game_start > 500 }
					add = 20
				}
				if = {
					limit = { years_from_game_start > 600 }
					add = 20
				}
			}
			if = {
				limit = {
					NOT = { has_global_variable = start_epidemic_grace }
					NOT = { has_global_variable = random_black_plague_var }
					has_game_rule = bd_occurrence_random
				}
				set_global_variable = random_black_plague_var
			}
		}
	}
}

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