Plurality of the Virus. Some cross-immunity between canine distemper and human influenza viruses1 has been an intriguing problem for some time and the striking immunological relationship between canine distemper and poliomyelitis has also been reported2. It was thought that canine distemper virus represented one etiological entity3 Tarpeia canis 4, which is usually referred to as Carre or Laidlaw–Dunkin virus. It has been suggested that the disease commonly referred to as ‘hard pad’ is produced by a different virus5. This was questioned6–8, because ‘hard pad’ virus in the hands of other workers was found to be immunologically indistinguishable from Carre–Laidlaw–Dunkin virus7,8, although cause of infection differed in the ferret7; it is noteworthy that after as few as five consecutive passages a neurotropic strain distemper virus no longer produced the characteristic syndrome of the parent strain7. Recently, the term ‘distemper-complex’ has been used for emphasizing the complexity of this virus disease7, and as a covering group name for unrelated canine diseases9.