Leukemia develops when the DNA of developing blood cells, mainly white cells, incurs damage. This causes the blood cells to grow and divide uncontrollably. The normal blood cells die while the abnormal cell continue to multiply in the bone marrow. These abnormal cells soon outnumber the normal blood cells and then obstruct their normal functions. The damage to DNA could be caused by: *physical agents such as exposure to ionising radiation *biological agents such as viruses e.g HTLV - Human T Lymphotropic Virus *genetics e.g patients with down syndrome have higher chances of developing a leukemia.