Anemia is a condition in which you don't have enough healthy red blood cells to carry adequate oxygen to your tissues. Having anemia may make you feel exhausted. There are many forms of anemia, each with its own cause. Anemia can be temporary or long term, and it can range from mild to severe. Signs and symptoms vary depending on the cause of your anemia. Initially, anemia can be so mild it goes unnoticed. But signs and symptoms increase as anemia worsens. Anemia occurs when your blood doesn't have enough red blood cells. This could result if: Your body doesn't make enough red blood cells; Bleeding causes you to lose red blood cells more quickly than they can be replaced; Your body destroys red blood cells.