There are two major routes of transmission: the faecal-oral route and the transplacental route. During acute disease as many as 109 TCID50 virus particles are shed per gram of faeces (Pollock 1982), and these may be passed 2–3 days post-infection, before symptoms become apparent (see Fig. 3). The minimum infectious dose for wild strains of CPV is unclear, although experience with an attenuated strain suggests that it may be a very small dose, since vaccination with this strain is accomplished by intramuscular inoculation with 16 TCID50 (Carmichael et al. 1983). The scale of virus shed during infection and the intrinsic stability of CPV particles must have contributed to the enzootic status of the virus. Mechanical (passive) transmission of CPV is also important: infective virus may be carried out on dogs’ hair and feet, on clothing and footware of dog owners, on fomites (such as feeding dishes) and by insects, notably flies. In the past this means of transmission may have been particularly significant at dog shows and in kennels where large numbers of dogs were gathered together. It must have contributed significantly to the high mortality rates recorded for young pups in the original pandemic.