Zinc is an essential mineral perceived by the public today as being of "exceptional biologic and public health importance", especially increasingly regarding prenatal and postnatal development.[6] Zinc deficiency affects about two billion people in the developing world and is associated with many diseases.[7] In children it causes growth retardation, delayed sexual maturation, infection susceptibility, and diarrhea.[6] Enzymes with a zinc atom in the reactive center are widespread in biochemistry, such as alcohol dehydrogenase in humans.[8] Consumption of excess zinc can cause ataxia, lethargy and copper deficiency.