Worldwide, the most common ailment of the blood is anemia, with the most common cause being parasitic infections such as hookworm, whipworm, or amebiasis. Anemia is a decrease in the normal level of red blood cells, or a less than normal hemoglobin level in the blood cells.

In developed countries, those suffering from anemia often don't realize it. This contributes their general weakness, fatigue, and poor concentration, all classic symptoms of anemia, in addition to stress, raising children, or poor diet. Some may attribute their shortness of breath during physical exertion to being out of shape. While any one of these symptoms can be caused by a number of physical ailments, when combined, they are often the warning signs of anemia.

There are three different classifications of anemia: microcytic, macrocytic and normocytic. Iron deficiency anemia is the most common type of anemia overall, with the primary cause worldwide being parasitic infestation, although there are other causes as well. In the United States alone, 20% of women of child bearing age have this type of anemia, and 2% of adult men have it. It can also affect children and infants, although these cases are less common.

Anemia caused by iron deficiency is due to the body not getting enough iron from food, or due to the body not being able to absorb the iron it is getting. When the body doesn't have enough iron, no matter the reason, it is unable to adequately replace hemoglobin losses that occur during menstruation or due to disease. This is because iron is a major component of hemoglobin, and when the body lacks iron, the hemoglobin level absorbed by red blood cells is greatly decreased.

In developed countries, iron deficiency anemia is generally caused by blood loss during menses and bleeding lesions in the gastrointestinal tract. In underdeveloped countries, is is most often caused by parasites.

Treatment for anemia depends on the cause, as well as the severity of the case. Iron deficiency anemia caused by blood loss during menses and poor nutrition is often treated with a combination of vitamin supplements and diet rich in iron. These treatments also work when parasitic infestation is the cause, although the parasites must first be removed or killed for them to work effectively.