Geography May Explain Adult Mortality from the 1918-20 Influenza Pandemic

Low exposure to H1-like viruses in adults could not alone explain the high total mortality in remote populations (up to 90%). A high concurrent disease load, crowding, low genetic variability, a lack of basic care, and infrequent exposure to other forms of influenza virus 1890-1917 may have played a role as well. This form of immunological cross-protection from previous exposure to A-type influenza viruses other than H1N1 can only be explained as a consequence of cellular immunity against internal proteins that show less inter-strain variation than the surface proteins.