Anemia continues to affect millions of people around the world despite increasing availability of iron. This presents a puzzle of sorts, as to why this conditon would continue despite an obvious and relatively cheap solution of iron supplementation.
A new study looks at anemia from an evolutionary perspective and provides some answers. The authors believe that anemia is a successful phenotype of sorts. As Tobias pointed in his comment, bacteria needs iron to proliferate. When humans switched to an agricultural economy, a number of new diseases due to crowding and sanitation came about. These included plagues, malaria, and tuberculosis. It turns out that those individuals who had lower levels of plasma iron most likely had higher survival rates through these diseases. Through the selection pressures of this period, individuals who were anemic were more likely to reproduce and pass their genes on.
So even though anemia is not an ideal state of health, it was optimal for survival against these diseases. The authors conclude that because of this, it will take continuing effort and a long period of time until anemia is lessened in the modern world.