Retroviruses are RNA viruses widely distributed in most vertebrate and in many non-vertebrate species. Among the retrovirus family, small ruminant lentiviruses (SRLV), Jaagsiekte sheep retrovirus (JSRV) and enzootic nasal tumor virus (ENTV) infect small ruminants. JSRV and ENTV are oncogenic retroviruses responsible for pulmonary and nasal adenocarcinomas, respectively, which can lead to the death of the animals in few weeks or months. SRLV are non-oncogenic and replication-competent lentiviruses, related to human immunodeficiency virus (HIV). SRLV are responsible for slowly evolving progressive degenerative diseases leading to the death of the host after few months or years. These viruses are transmitted through the colostrum or milk and aerosolization of infectious particles. This review focuses on the biology of the small ruminant retrovirus and on the main mechanisms of disease induction.