Historically, the term "slow virus infections" was coined for a poorly defined group of seemingly viral diseases which were later found to be caused by several quite different conventional viruses, also unconventional infectious agents. They nevertheless shared the properties of causing diseases with long incubation periods and a protracted course of illness, affecting largely the central nervous and/or the lymph system and usually culminating in death. The slow virus concept was first introduced by the Icelandic physician Bjorn Sigurdsson (1913–1959) in 1954. He and his co-workers had made pioneering studies on slow diseases in sheep including maedivisna and scrapie. Maedi is a slowly progressive interstitial pneumonia of adult sheep while visna is a slow, progressive encephalomyelitis and the same virus, belonging, to the lentivirus subgroup of retroviruses,...