Experts estimate that only about a third of humanity was infected by the Spanish Flu.

That doesn't say anything about the rest. Were they immune somehow or just lucky to avoid exposure? We don't know.

Remember some of these ridiculous sounding potential mortality estimates when H1N1 swine flu was starting to go pandemic? Well, take the world's population, take a third of that, and then multiply by 3%. That's roughly the Spanish Flu's experience translated to today's global population.