Each flu strain is different. The 1918 flu is different than the swine flu which is different from the Asian flu which is different from the bird flu. Thus, an immunity to one strain of flu might provide some sort of immunity to different types of flu, it does not guarantee an immunity to all types of flu.
If you've never had the flu, you're just a lucky guy.
1918 flu strain was a subtype of avian strain H1N1, in other words.... bird flu.