What causes anemia?
I am just going to give a brief answer, other members may expand and add more if they so wish.
It is caused by a lack of iron (Fe) in the diet.
Iron is required to bond onto the pigment haemoglobin in the blood which carries oxygen around the blood for transport into cells.
Hence a lack will result in constant tiredness, fatigue and lethargy, breathlessness, etc.
It can indeed be caused by an unbalanced diet, but may also be caused by genetic reasons/inheritance.
Anemia means without blood. Actually it means low level of iron in the protein hemoglobin packed in the red blood cells.
It call also be caused by a number of other factors: loss of blood, cancer of the colon, parasites such as hookworms, lack of vitamin 12, folic acid, intrinsic factor (needed to absorb B12). Low thyroid hormone levels, liver disease, bone marrow failure.
As you can see, it is a disease with many reasons.