Infection with one strain of flu doesn't provide immunity to the other strains of flu, even ones in the same family (different H1N1s, etc). That's why you need

Infection with one strain of flu doesn't provide immunity to the other strains of flu, even ones in the same family (different H1N1s, etc). That's why you need a new vaccine every year. That doesn't mean herd immunity wasn't eventually reached for the strain that killed people in 1918. If everyone is either dead or recovered, the virus is simply not going to spread like it did when it was new.