a group of virus-induced transmissible diseases of chickens and some other birds in which there is lymphoid cell infiltration or formation of lymphomatous tumors in various tissues and organs; the two principal diseases are: 1) the avian leukosis-sarcoma complex-induced lymphoid leukosis, involving the bursa fabricius and various visceral organs, that is associated with viruses of the family Retroviridae; 2) Marek’s disease, caused by avian herpesvirus 2 and involving primarily the peripheral nerves and gonads and, to a lesser and more variable extent, other visceral organs, skin, muscle, and the eye. Variability of lesion site prompted other names for avian lymphomatosis, such as big liver disease, ocular lymphomatosis, visceral lymphomatosis neurolymphomatosis gallinarum, and fowl paralysis. Syn: fowl lymphomatosis.