Causes of Anemia
A complete list of all the causes of anemia can fill several chapters in a textbook, but all causes can be grouped into one of three basic problems: either the body is unable to manufacture enough red blood cells, those cells are being destroyed before they should be, or red cells are being lost through bleeding.
Blood loss can be sudden, or slow and insidious. In the first few months of life, a baby with a milk protein allergy can become anemic from inflammation and bleeding within the gut. Parents may see flecks of blood in their baby's diaper, but not always. A single intestinal polyp can cause bright red blood loss from the gut. Older children with Crohn's disease can suffer months of intestinal blood loss before the problem is diagnosed.
A second cause of anemia is premature destruction of red blood cells. The typical lifespan of a red cell is about 120 days. Worldwide, millions of people with malaria suffer sometimes massive destruction of red blood cells as the malaria parasite, which grows inside red cells, breaks free. Newborns born to mothers with an "incompatible" blood type can have their red cells destroyed when antibodies from the mother cross the placenta and attack those cells. Drugs and certain infections can create a similar situation in older children.
Finally, anemia can be the result of a body's inability to make enough red blood. Producing blood is the job of the bone marrow. In leukemia, the marrow is so crowded by cancerous white blood cells that red cells cannot be made. Certain viruses or drugs can temporarily suppress blood production. In other conditions, a mutation in the DNA codes causes misshapen hemoglobin molecules that do not function well. Sickle-cell anemia is an example of this.
Lastly, if the body lacks the essential "ingredients" needed to make red blood, it can't. Vitamin B12 and folate deficiencies lead to anemia, and—this is the big one in pediatrics—so does iron deficiency. In our overall well-fed population, B12 deficiency is rare and seen mostly in children fed strict vegan diets. The same is true for folate deficiency, though the cause is usually that children are being fed goat's milk.