Sex differences in the immune responses to influenza viruses have not been systematically examined [117] . Using data from other virus-host systems, several immunological, hormonal and genetic mechanisms have been identified as being differentially expressed between the sexes and altered during the course of pregnancy, which may account for male-female differences and pregnancyassociated increases in the severity of 2009 H1N1. Generally, women mount higher immune responses to viral infections [118] . Heightened antiviral immunity in women is beneficial for virus clearance, but may be detrimental if it becomes excessively high or prolonged, leading to pathology and even death. Over the course of pregnancy, inflammatory and antiviral immune responses are suppressed which can alter responses to viruses, such as influenza.