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This leads to problems caused by the lack of normal cells doing their job.
Without adequate white cells, infections occur which are often fatal.
Without platelets, fatal bleeding (such as bleeding into the brain) may occur.
Without erythrocytes (red blood cells) anemia occurs, which keeps oxygen from getting to the organs.
Treatment for leukemia usually includes chemotherapy, which kills the leukemic cells, but also kills normal ones, sometimes leading to fatal infections due to lack of white cells (red blood cells and platelets can be transfused, so fatal complications from the lack of those can usually be prevented).