Bovine immunodeficiency virus (BIV) is a retrovirus belonging to the Lentivirus genus. BIV shares morphologic, antigenic and genomic characteristics with other lentiviruses. It is similar to the human immunodeficiency virus (HIV). It causes persistent infection and slow progressive disease in cattle and probably in sheep. As target cells of the virus are leukocytes, the type of which is unknown, perturbations of the immune system are expected. Consequently, BIV may potentiate the occurrence of secondary infections and play a role in retroviral, multiple infections.