The causes of diffuse large B-cell lymphoma are not well understood. Usually DLBCL arises from normal B cells, but it can also represent a malignant transformation of other types of lymphoma (particularly marginal zone lymphomas) or, in rare cases termed Richter's transformation, chronic lymphocytic leukemia. An underlying immunodeficiency is a significant risk factor for development of the disease. Infections with the Epstein–Barr virus (EBV), Kaposi's sarcoma-associated herpesvirus, human immunodeficiency virus (i.e. HIV), and the "Helicobacter pylori" bacterium are also associated with the development of certain subtypes of diffuse large B-cell lymphoma. However, most cases of this disease are associated with the unexplained step-wise acquisition of increasing numbers of gene mutations and changes in gene expression that occur in, and progressively promote the malignant behavior of, certain B-cell types.