The bone marrow, which is the spongy liquid inside your bones, makes stem cells. These stem cells develop into the 3 main types of blood cells: red blood cells, white blood cells, and platelets. Red blood cells are transporters. They carry oxygen from the lungs to the cells and carbon dioxide from the cells to the lungs. White blood cells are part of the body's immune system. Platelets help the blood clot.

The bone marrow in a healthy person makes blood cells all the time to replace those that die. A red blood cell lives for about 120 days. A white blood cell often lives for less than a day. A platelet lives for about 6 days. In a person with aplastic anemia, the body slows or stops making the normal amount of blood cells. Or the blood cells that are made are damaged or defective. Researchers don't know what causes this in most cases of aplastic anemia. Occasionally, the disorder is caused by certain medicines such as the antibiotic chloramphenicol. Researchers don't know why this happens.

Red blood cells contain hemoglobin. Hemoglobin carries oxygen from the lungs to all cells in the body. Red blood cells also remove waste carbon dioxide from the cells and carry it to the lungs to be breathed out (exhaled). If the body has too few red blood cells, too little oxygen reaches the cells. This condition is called anemia. It causes a person to feel tired. The heart must pump harder to make up for the lack of oxygen being delivered to the cells.

White blood cells fight infections. If you have too few white blood cells, you can't fight off infections as well. You may become ill more often. A low white blood cell count also means illnesses last longer or are more severe than in a person with a normal count.

Platelets help the blood to clot. Having too few platelets raises the risk of bleeding or bruising. With too few platelets, it may be difficult to stop bleeding once it starts. Bleeding can occur even from a mild injury that normally would not be noticed.

Aplastic anemia is rare in the U.S. Anyone at any age can get this disease, but it’s more common in these age groups.

A person with the hereditary form of aplastic anemia is born with the illness. A person with the acquired form—the more common type—may develop the illness after exposure to something in the environment. These triggers are thought to include toxic chemicals such as pesticides, chemotherapy medicines used to treat cancer, radiation, autoimmune diseases such as lupus and rheumatoid arthritis, and viral infections such as hepatitis, cytomegalovirus, and HIV. But often healthcare providers don't know the cause of this type of aplastic anemia.

These treatments include blood transfusions, medicines to stimulate the bone marrow, immunosuppressant medicines, and bone marrow or stem cell transplants. Treatment depends on the cause and severity of the illness. People with mild or moderate aplastic anemia may not need treatment. A healthcare provider will check blood counts often to make sure the illness is not getting worse. Treatment is needed when a person has severe or very severe aplastic anemia. Many people with aplastic anemia can be treated and some can be cured.