The flu regularly migrates between pigs, chickens and humans. Whenever it does, the poorly adapted flu is unusually lethal.

The misnamed Spanish flu seems to have killed by triggering a strong immune response. That response is called a cytokine storm. Which means that the stronger your immune system, the more likely you were to be hit hard. It therefore killed the reverse of the groups that normally get killed by diseases.

There is also some evidence that people who experienced the "Russian flu" of 1889-1890 had some level of protection from the Spanish flu.