As CEACAM1 is the major CEA family member upregulated by IFN-γ and the receptor is targeted by other respiratory bacteria, including H. influenzae and M. catarrhalis (Virji et al., 2000; Hill and Virji, 2003), increased susceptibility to all these opportunistic bacteria following viral infection may be due to the upregulation of CEACAM1; a hypothesis that is under investigation. This evidence concerns the gene CEACAM1 and viral infectious disease.