A further confounding factor is that the number of viral particles and of genomes detectable by RT-PCR greatly exceeds the number of infectious units measured by plaque formation or cytopathology (TCD 50 or MPNCU). Isolation of more than one type of poliovirus from a plaque in a culture that received mixed inoculum has been reported by Teunis et al. (2005) , but this represents a relatively small potential disparity compared to the numbers of virions often shown to be present in a suspension. Direct particle counts by scanning electron microscopy gave mean particle:PFU ratios of 448 for a vaccine strain of PO1, and 38 for an ostensibly virulent strain of CB5 (Heinz et al. 1986 ).