My ME was staged, so I had 2 significant onsets. Although I only recognized my mild onset upon looking back from my severe onset and realizing that I had been experiencing ME to a lesser degree in those years. My severe onset was a more horrific amplified version with even more added symptoms than my mild onset. But I could see the progression upon looking back from severe.
Southern Hemisphere.
Mild onset 1995, May (last month of autumn and getting chilly weather). Unusual flu with extreme dizziness which I had never experienced before in my life. Very sore throat.
Southern Hemisphere.
Severe onset, bone aching flu, 2001, September (first month of spring) interestingly though, my sleep had packed up in May of 2001 same month as my above mild onset. I was waking at 3-4am every morning 'wired and tired' feeling absolutely awful - something very wrong. It was a very quick and horrible downward spiral to severe ME from there.
I do think my ME was caused by an infection at 20 years old and combined with flu years later caused ME symptoms to appear and progress. So a long staged illness.