To determine the ability of PR8 PB1-F2 variants to enhance secondary bacterial pneumonia, we infected mice with a non-lethal dose of 15 PFU of virus per mouse, or treated with PBS as a control, and 7 days later challenged them with a non-lethal dose of 100 CFU of SPn per mouse or with PBS (Fig. 5b–d). Here, SPN is linked to bacterial pneumonia.