One interesting thing I read about the 1917-1918 worldwide flu pandemic was this. Older people didn't catch it, and didn't die from it, at anywhere near the rates of younger people (18-40.) The reason, it's thought, is because in the late 1800s there was a "mini-pandemic" in parts of Asia, Europe, and the US. Younger people who were exposed in the late 1800s and survived grew up to be the older adults who were more resistant. Those born *after* the circa 1880s pandemic had no resistance to the 1917-1918 flu, and thus had more deaths.
It seems to me that allowing oneself to get the flu as a younger, healthier person would build antibodies that would still work when one *was* older and sicker.
Even in 1918 there were two waves of that flu --- the first wave was a milder version --- I wonder if those who caught that and developed antibodies were protected from that second more virulent wave that came in the fall?
It's a good idea to have your natural immunity built up and that gives you better cross-immunity. I also wonder with that flu in 1918 if there were other factors that caused the deaths -- secondary diseases --- possibly spread by health care workers? Overcrowded conditions? Remedies that actually made the disease worse?
The problem these days though is that each year a newer an more resistant strain mutates, therefore any immunity you may have built up in previous years won't protect you. But by the same token, I don't get the shot simply because I've known too many people who have gotten the vaccine and gotten the flu in spite of it (or maybe because of it?)