Zinc deficiency can occur in soil, plants, and animals. In animals, including humans, it is defined either qualitatively as insufficient zinc to meet the needs of the body and thereby causing clinical manifestations, or quantitatively as a serum zinc level below the normal range; however, serum zinc is not a reliable biomarker for zinc status in humans. Zinc deficiency affects about 2.2 billion people around the world.Zinc deficiency in humans results from reduced dietary intake, inadequate absorption, increased loss, or increased use. The most common cause is reduced dietary intake; as much as 25% of the world's population is at risk. Increasing the amount of zinc in the soil and thus in crops is an effective preventative measure. Zinc plays an essential role in numerous biochemical pathways. It affects many organ systems, including the skin, gastrointestinal tract, central nervous system, and immune, skeletal, and reproductive systems. A lack of zinc thus has numerous manifestations, the most common of which are an increased rates of diarrhea, pneumonia, and malaria.