Taking care of yourself is half the equation. The other half is exposure. IMO one can be healthy as a horse and can still catch a cold or the flu. True they have a better chance of having a lighter bout, but I've never heard valid medical studies that proved those with good immune systems don't get sick. Don't get sick as often, yes. Don't get sick at all, no.
Due to the nature of my workplace (hospital) I have had yearly flu shots for the past 25+ years at least. I haven't had a "classic" heavy-duty flu in all that time, but I have had light versions a few times.
You may feel punky for 2-3 days after a flu shot. You may have a sore arm, a slight fever and systemic malaise. This is NOT the flu, and you are NOT "sick." It's a reaction to the inoculation. I would rather have that than actual flu, which customarily lays people out flat in bed for 10 days to two weeks, literally unable to function.
And/or, yes, in rare cases one can be incubating a cold or the flu already, in which case it *appears* that the flu shot "gave" them the flu - especially if you get the shot late in flu season.
I had the first cold I've had in 2-3 years, last month. I had been going through a physically & emotionally stressful 6-week period and wasn't surprised that my immune system had karped out. But I hadn't had a cold in so long I forgot how miserable it was :-(
Life without Mexican food is no life at all