Anemia describes one of a number of diseases that leads to a shortage of red blood cells in the blood. It has a number of causes, which fall into four general categories:

- Shortage of material to create new blood cells (e.g. iron deficiency).

- A malfunction of the bone marrow that creates red blood cells, such as is common with several forms of cancer (e.g. aplastic anemia).

- The destruction of red blood cells by a foreign material.

- Genetic conditions that causes the cells to no longer be able to carry oxygen (e.g. sickle-cell anemia)

If untreated, anemia is invariably fatal. Many forms of anemia can be treated by treating or managing the underlying condition. However, when red blood cell counts fall below a certain level, a blood transfusion is usually required to replenish the level of red blood cells in a patient.

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