a group of conditions (e.g., lymphoid, erythroid, or myeloid leukosis) that occur chiefly in chickens and are characterized by an abnormal proliferation of myelopoietic, erythropoietic, or lymphoid tissues; etiologic agents are a group of closely related viruses in the family Retroviridae, and the conditions are transmissible. See Also: avian leukosis-sarcoma complex. Syn: fowl leukosis, leukemia of fowls.