Adults catch real flu about once every five years, scientists calculate, based on a field study in China.

https://www.bbc.com/news/health-31698038

The CDC estimates around 30 million symptomatic influenza infections in a year; in recent years, more[1].

And roughly three-quarter of infections are asymptomatic[2], so ~120 million total cases annually.

[1]: https://www.cdc.gov/flu/about/burden/past-seasons.html

[2]: https://www.sciencedirect.com/science/article/pii/S221326001...

Flu is kinda like COVID: a significant percentage of infections are without symptoms.

Lots of estimates, I usually see it as around one-third of cases having no symptoms: https://www.ncbi.nlm.nih.gov/pmc/articles/PMC2646474/

I’ll take a slight leap and guesstimate this means a lot of flu infections are around the symptom level of a mild or bad cold as well.

When you know you have the flu, it’s probably the flu, but it’s much harder to self-determine that you don’t have the flu.

I read once that many cases of the flu are really food poisoning. I got H1N1 and felt like I was dying for a week. Looking back I now think it's the only time I've ever really had the flu.

Correct. I'm 36 and unless it was mild or asymptomatic, I'm sure I've never had influenza.

When people exaggerate a common cold into "the flu" I respond with equal exaggeration by insisting I take them to the hospital.

Influenza can be mild, even asymptomatic, so unless you are really able to test; you probably don't have good numbers for how often you've caught the flu.

Influenza is rare? Maybe rare for you if you habitually get the flu shot twice a year, but even then the efficacy of the flu shot can be as low as 50%. Influenza is very common, most people have experienced it.