There can be several causes for anaemia and varying types, however the most common is a lack of iron, which leads to reduced haemoglobin levels and red blood cell production.
This could happen all of a sudden, e.g. via blood loss due, or it could be due to a longer-term issue such as a dietary deficiency. There are also certain periods in which there is a strong tendency for anaemia to form, such as pregnancy.
In addition to iron, RBCs need Folic acid and vitamin B12, in order to reproduce effectively and in sufficient supply.
Genetic conditions, which lead to anaemia, also exist, such as Sickle-Cell disease (SCD). SCD is characterised by RBCs that assume an abnormal, rigid sickle like shape. Life expectancy is shortened significantly in people suffering from this disease.