IFNA1 and infection: These mechanisms are not fully effective however, as demonstrated by the fact that PIV5 (like other negative strand RNA viruses) produces larger plaques in ‘IFN-compromised’ cells than in ‘IFN-competent’ cells — i.e. at some point at least some virus-infected cells must have secreted IFN thereby slowing the spread of infection (data presented here, and Young et al., 2003).