1950年Sigurdsson报导冰岛绵羊中的几种“慢”病毒感染,其中有些仅侵犯中枢神经系统。他对慢感染所下的定义是:病原体在几个月或几年中缓慢繁殖,引起进行性临床症状,通常具有很长的潜伏期,感染往往局限于单一器官。感染所伴有的发热、血液和组织改变在这些疾病中常缺如。 In 1950 Sigurdsson reported several “slow” viral infections in Icelandic sheep, some of which only invaded the central nervous system. His definition of a slow infection is that the pathogens multiply slowly over months or years, causing progressive clinical symptoms, often with a long incubation period, and the infection is often confined to a single organ. Fever accompanied by fever, blood and tissue changes often lack these diseases.