slow virus, technically a virus, such as a lentivirus, that causes symptoms in an infected host long after the original infection and progresses slowly. Although many viruses fit this description, the term slow virus is usually reserved for the first recognized lentiviruses, such as the virus that causes visna (a disease of sheep). A slow virus was proposed as a cause for those diseases now generally recognized as prion diseases (e.g., Creutzfeld-Jacob disease and scrapie).