Genetic factors thought to cause leukemia include Li-Fraumeni syndrome and Down syndrome, notes MedicineNet. Cigarette smoking, exposure to radiation, human T-cell leukemia virus type 1 and blood disorders called myelodysplastic syndromes are other risk factors. Having risk factors for leukemia does not mean that the patient will get leukemia.
As of 2015, the cause of leukemia is unclear, but scientists think that it occurs because of mutations in the DNA of some blood cells, causing cells to divide and grow too rapidly, reports Mayo Clinic. These abnormal cells don't die the way normal cells do, resulting in fewer healthy cells over time. This causes symptoms of leukemia, such as excessive sweating, swollen lymph nodes, tiny red spots in the skin and recurrent nosebleeds.