Anemia is a condition that develops when your blood lacks enough healthy red blood cells. These cells are the main transporters of oxygen to organs. If red blood cells are also deficient in hemoglobin, then your body isn't getting enough oxygen. Symptoms of anemia -- like fatigue -- occur because organs aren't getting enough oxygen. Anemia is the most common blood condition in the U.S. It affects about 3.5 million Americans. Women and people with chronic diseases are at increased risk of anemia. Follow the links below to find WebMD's comprehensive coverage about what causes anemia, what it looks like, how to treat it, and much more.