Mouse plagues have occurred several times throughout parts of Australia since wild house mice (Mus musculus) were introduced by European colonists along with the First Fleet in 1788. Australia and China are the two countries in the world where plagues of mice are known to occur. Mouse plagues occur in southern and eastern Australia, usually in the grain-growing regions, around every four years. Aggregating around food sources during plagues, mice can reach a density of up to 3,000 per hectare (1,200/acre).