This ME Alliance report by Charles Shepherd, et al., gives a figure of 75% for sudden onset "often dating back to an acute viral infection." Not all sudden onset cases have a viral suspect, but my impression is that those cases that do account for the majority of that 75%.
my debut was sudden, probably viral, with moderate sore throat, low-grade fever (37.2 °), sweating between neck and neck naturally fatigue;
with each passing week came the digestive disorders, concentration / memory problems, headaches, earaches, vulnerability to cold and the sore throat / voice loss that comes and goes ...., joint pain / muscle.
I doubt very much the ME Alliance relationship, other potential causes of ME than the hypothetical viral
because I think that the cause of ME is a germ or a family of germs (viruses) still unknown why some viruses "known" may be reactivated during the crisis of the sick and other germs are easily contracted from sick ME .....