Lymphosarcoma is the pathologic term that describes a fatal cancer of lymph glands or lymphoid tissue. Other terms that have been used to describe this condition in cattle are leukemia and leukosis. The designation of leukosis was given by European veterinarians who first recognized the disease in an area near the Baltic Sea in the latter part of the 19th century. During the next few decades it became clear that the disease was spreading westward across Europe and this epidemiologic observation suggested the tumor was due to an infectious agent. Although death losses from lymphosarcoma were not severe, many European countries already had eradicated the more lethal infectious diseases, or at least reduced them to a very low incidence, so they were interested in doing whatever they could to maintain their high standards of animal health.