The DISC virus infectious titre (TCID 50 ml −1 ) was estimated in CR1 cells on 96-well plates infected with serial tenfold dilution's of virus material. The CPE was read 3-4 days post infection using a TCID 50 ml −1 method (Reed and Muench, 1938) . The TCID 50 ml −1 assay is used with a subsequent conversion to pfu ml −1 as described by Dougherty (1964) . The TCID 50 dose can be measured by a number of means and the method of Reed and Muench (1938) was employed in this case. This TCID 50 is the usual end point for many virus assays. The conversion to pfu ml −1 takes into account the distribution of virus in the suspension according to Poisson's Law. According to Poisson distribution the TCID 50 would contain 0.69 infectious particles per unit volume. Therefore this correction factor can only be applied when chance distribution of virus in suspension occurs (Dougherty, 1964) .