“…In 1961, Richardson, supported by Waksman and Adams (1962a, b), put forward the hypothesis that progressive multifocal leucoencephalopathy is a viral disease which affects individuals with deficient immune response due to their primary disease, most commonly a disorder of the reticuloendothelial system, i.e., lymphatic leukaemia or Hodgkin's disease. Since then the viral theory has received the most support and discussion, although (Headington and Umiker, 1962;Deep et al, 1964), and excess activity of the reticulo-endothelial system (Weinstein et al, 1963). Cavanagh, Greenbaum, Marshall, and Rubinstein (1959) also suggested a disturbance involving lipid metabolism, in particular that of sphingomyelin.…”