One cause of low red blood cell count is pregnancy, but this is normal, according to Mayo Clinic. However, among the many other conditions and diseases that can cause a low red blood cell count are cancer, aplastic, vitamin-deficiency and iron-deficiency anemia, internal or external bleeding and porphyria.
Other causes of low red blood cell count are lead poisoning and sickle cell anemia, states Mayo Clinic.
Aplastic anemia is a disease where the bone marrow doesn't make enough red blood cells, says the National Heart, Lung and Blood Institute. This disease can be inherited or its cause can be unknown.
Porphyria is a rare disease where the enzymes that create porphyrin and heme don't work well, according to Wikipedia. Porphyrin and heme are necessary for the proper functioning of red blood cells. Lead poisoning results in low red blood cell count because lead weakens the membranes of the red blood cells and creates radicals that interfere with DNA.
Sickle cell anemia causes deformed red blood cells that can't carry oxygen the way they should, according to Mayo Clinic. These cells clog blood vessels and can block the flow of blood. This can lead to such complications as strokes and organ damage even in young children.