In urban communities, less than 1 in 100 inhabitants died from Spanish flu in 1918, but in isolated communities up to 9 out of 10 died. An important explanation for the differences is due to different exposure to influenza in the decades before the Spanish flu came. Those living in urban communities probably had a higher degree of pre-existing immunity that protected against illness and death in 1918 than those living in very isolated rural areas.

Source: 1918 influenza pandemic (Spanish fl...y between urban and isolated rural areas