Immunology and influenza
Pregnant women and people with diseases or drugs which supress their immune system are more likely to catch Influenza and having caught it more likely to be more ill than average.
Pregnancy necessarily involves a reduction in immune activity of some sorts, else the mother would reject the foetus.
The virus of the 2009 pandemic H1N1 Influenza, seems, particularly in Australia, to be unusually bad for pregnant women.
In the immunocompetent, particularly the young, and particularly those (who are generally also young) who have not previously been exposed to the antigens of the virus, a mechanism of very rapid death which would account for cases observed in the 1918 pandemic, is a cytokine storm.
Immunosuppression actually protects to a degree against the overwhelming immune activity of a cytokine cascade or storm.
At first encounter this apparently paradoxical situation may puzzle people.