To independently estimate the latent infection period for the two A/Brisbane/59/2007 influenza virus strains, we performed a single-cycle viral yield experiment. Single-cycle experiments were performed at an MOI of 1 such that most cells would be infected simultaneously and pass through the phases of latency and viral release at the same time. Therefore, the observed virus production of the cell culture can be considered roughly proportional to that of an individual cell. The results of two independent experiments for each strain are shown in Figure 7A ; one experiment shows the viral titer over one full day post-infection and the other over only 14 h but with more frequent sampling. For each replicate, the viral titer of each strain was observed to grow rapidly after 4 h postinfection, with the WT viral titer reaching a plateau at approximately 8 h post-infection and that of the H275Y mutant reaching a plateau between 10 h and 14 h post-infection. Although the viral titer data in each replicate followed a relatively smooth curve, the inter-replicate variation was quite large, with peak virus titer varying from 2|10 3 PFU mL to almost 10 5 PFU mL. It is also notable that all of these peak values were well below the values seen in the multiple-cycle viral yield assay (Figure 3) , by a factor of * > 1000. Both of these features could be explained by the action of a relatively large defective interfering particle population [43, 44] within the viral stock, which is not uncommon for the influenza virus [45] .