My illness was sudden in onset, though there have been different stages. But all of the changes were sudden.
Went to Jamaica, 2 weeks after getting home was suddenly lightheaded one morning, felt like I had a fever, and went to school nurse and found my temperature dropped about 2 degrees, was lightheaded for 2 years. Temperature is still always low, 7 years later. 95-97 degrees. The day I have a normal temperature I will declare victory, because that started the first day of weirdness, and hasn't gone away since.
Went to India a couple years later, sudden onset of CFS symptoms one night. Didn't get better until I came back to the states, when I recovered to about 80% health. Then a year later after working a job in the freezing cold for 4 days, I got a strange cold, the CFS symptoms came back same as in India, and 4 years later and they still haven't gone away.
All these changes happened suddenly, and then stayed constant for periods of time until the next sudden change. Except for the last 4 years, in which I've slowly gotten worse without hardly any periods of improvement.