As a kid I had the flu a few times and the difference between flu and a cold is with the flu it seems to affect the optic nerves of the eye making it hurt to look sideways.
I asked my doctor recently about getting the flu needle as I had just spent 12 days getting over the sniffles and over a month later I still have bronchitis and he said the best way to get immunity from the flu is to get the flu. Needless to say I am not going back to that doctor again.
Perhaps he read this paper on the 1918 flu where it wasn't just the young and elderly who copped it, it was the healthy ones who were in their prime and their own bodies went in to overdrive to fight it.
https://shorthand.uq.edu.au/small-change/fifty-million-dead/