Leukaemia is a progressive, malignant disease of the blood-forming organs, characterised by distorted proliferation and development of leukocytes and their precursors in the blood and bone marrow. It can be classified as acute or chronic, according to the degree of cell differentiation (not the duration of disease), and as myelogenous or lymphocytic, according to the predominant type of cell involved (myeloid or lymphoid). Several sub-types have been identified. The exact cause of leukaemia is unknown, but genetic and environmental risk factors have been identified for many sub-types. Typical signs and symptoms might include fatigue, weight loss, fever, pallor, ecchymoses, petechiae, dyspnoea, dizziness, palpitations, bleeding, and recurrent infections, although each sub-type has its distinguishing features. Definitive diagnoses often require bone marrow biopsy and/or blood analysis.