Gradual onset sometimes hard to describe
In my case, I went through a period in the early 90's where I would kept thinking that I had something like AIDS, because I caught one bug after another for a period of almost a year.
Before that, possibly related, I had gone through a round of prednisone. I was an actor then and had lost my voice due to overuse and swelling of the vocal chords. I needed to finish the run of a play, so the doctor loaded me up with the steroids, which I found extremely unpleasant. So I quit taking them early. Soon thereafter came a bacterial infection and dose of flagyl. My digestive track was totally destroyed -- food passed right through and I became a skeleton and could barely stand.
All this I recovered from eventually but not long after developed what some would call fibromyalgia -- insomnia, pain, the usual. But I could still exercise. And when I did manage to get sleep, I could think better and seemed more like my old self. This went on for 8 years. Occasionally I would get bad pain attacks, was evaluated for rheumatoid arthritis and told I did not have it.
Then I caught a particularly nasty flu that kept me in bed 3 weeks. Things intensified after that. Soon I felt poisoned, constantly dizzy, could not think and speak at the same time, the usual cognitive nightmares, heart problems, etc. Got diagnosed, went on a TCA for sleep -- worst of the "poisoned " feeling went. Gradually have declined -- with a few good spells now and then which I ruined by doing too much. Am now about a 4. Was a 2 for a couple of months recently but began isoprinosine and am a little better.
Kind of a round-about gradual onset I guess.