Aplastic anemia happens when your bone marrow stops making enough blood-forming stem cells. Most experts believe aplastic anemia occurs because your immune system attacks and kills your stem cells in your bone marrow. This causes you to have low blood counts for all three types of blood cells.
Low blood counts result in symptoms such as fatigue, tiredness, bleeding, bruising and a higher risk of infection. The disease can be classified as moderate (MAA), severe (SAA) or very severe (VAA) depending on how low your blood counts are.