Bovine viral disease unit mainly studies viral diseases of bovine, horse, sheep and goat with virological approach. Current major research activities in the unit include enzootic bovine leukemia (EBL) and bovine viral respiratory diseases. Bovine leukemia virus (BLV), the causative agent of EBL, integrates into the genomic DNA of B-lymphocytes as the provirus, and then a small proportion (less than 5%) of BLV-infected cattle develop leukemia (lymphosarcoma) after a long latent period. Although the number of EBL cases has been increasing, the implementation of effective preventive measures is challenging because of high prevalence of BLV infection among cattle population in Japan. We are focusing on useful on-site markers as the indicator of EBL for effective control in affected farms. Etiologic viruses of bovine viral respiratory diseases, which include infectious bovine rhinotracheitis virus and bovine respiratory syncytial virus, spread rapidly among cattle via contaminated respiratory secretions, and cause serious respiratory disease complex not only in young cattle but also in adult cattle. We are working on the development of effective diagnosis and prevention methods by investigating genetic characters and pathogenesis of etiologic viruses, disease prevalence, transmission mechanism, and risk factors for disease onset and exacerbation.