G. Petursson and Rikke Hoff-J0rgensen The concept of slow viral infections was first put forward in 1954 by Dr. Bjorn Sigurdsson, an Icelandic physician who had been studying some sheep diseases which were introduced into Iceland with the importation of a foreign breed of sheep in 1933. Sigurdsson's main criteria for defining slow infec tions were a very long initial period without clinical signs lasting months or even years following infection and a rather regular protracted, progres sive course, once clinical symptoms had appeared, usually ending in serious disease or death. Sigurdsson...