I had a bad case of the flu back in the 80's, which kept me down for two weeks. I would feel better during the day, but be down in the dumps each morning.
Just two weeks ago, I came down with what must have been flu for the first time since then, although it wasn't quite as bad. It did come on very suddenly, and in fact it was a single moment when I sneezed, ( not unusual for me, ) but I knew at that moment that I was sick. I was spiking a fever up to 102+, but Advil helped. For three days, I slept and was awake, for a few hours at a time, without regard to the time of day. I recovered very well, but as is typical of the flu, it seemed to linger. I thought I was over it but I would be sleepy all day, and like that.
I haven't ever had a flu shot, but I did let the doctor talk me into a pneumonia vaccine just recently, ( since I'm old, you know, ) so I blame that ... 1/2 :-)
WHat I had noticed abou the flu is that it makes one sick in a way that is not similar to a cold. It’s basically debilitating, and the onset of it is like being hit by a train.
I never do flu shots either, I’m basically scared of them, and since that one case of flu, I have never had it again.