The Bovine Leukosis Virus (BLV) belongs to the genus ‘Delta-Retrovirus’ within the family of ‘Retroviridae’. The BLV causes persistent infection in cattle and is the causative agent of Enzootic Bovine Leukosis (EBL). The virus primarily infects B lymphocytes. In infected animals, provirus can also be detected in macrophages and endothelial cells. Clinically inapparent courses may persist life-long or develop into other clinical forms of BLV manifestation. Persistent lymphocytosis is a benign, polyclonal proliferation of the peripheral blood lymphocytes which is characterized by an increase in the number of circulating B lymphocytes to between 40 and 80% of the lymphocyte population. Up to two years after infection, infected animals rarely develop persistent lymphocytosis. After 3 to 6 years in contrast, persisting lymphocytosis occurs in 30 to 70% of the naturally infected cattle depending on genetic and environmental factors. As a rule, animals with persistent lymphocytosis do not show any health problems in association with the disease. Tumorous leukosis is characterized by the occurrence of malignant lymphoma. Approx. 1-10% of the BLV infected animals and 10-30% of animals with persistent lymphocytosis develop the tumorous form.