There is also post viral fatigue, which is much more minor than M.E. but can feel wretched for weeks.
Sparklefizz, there is nothing minor about ME, I have a cousin who has had ME for over 40 years. At one time she was in a wheelchair, although that phase has thankfully passed, but her house has been adapted so she can sleep downstairs and there is a down stairs bathroom. You have my sympathy.
I have been prone in the past to post-viral fatigue, touch wood, I seem free of it at the moment. The worst I had, it lasted over 6 months, but usually it was 'only' 6 - 8 weeks.