A condition that arises when either there are too few erythrocytes (red blood cells), the erythrocytes do not contain sufficient amounts of haemoglobin, or the erythrocytes are abnormal in other respects. Anaemia often results from loss of blood or from a deficiency in the factors necessary to synthesize haemoglobin (e.g. iron) or erythrocytes (e.g. folic acid and vitamin B₁₂). Increased destruction of erythrocytes may be induced by certain drugs or severe infection, and an abnormal form of haemoglobin results in sickle-cell anaemia (see polymorphism).