One theory about the 1918 flu us that it hit young people harder because older people had encountered a similar flu already, before the younger generation were

One theory about the 1918 flu us that it hit young people harder because older people had encountered a similar flu already, before the younger generation were born. But in general, being exposed to (say) seasonal flu every year might not help you at all when a new strain of pandemic flu shows up. The 1918 flu tore through young healthy people.