I thought that I would relate my experience. My 7-year old son got it and was sick from Wed-Sat, and getting better Sunday, fine Monday and back at school, fever gone Friday night (high of 104 Friday).
My wife and I got cases that were never very severe, and I never really had a fever. However, for us the diseases lingered. I did not workout for 16 days, although I rode to work and back 2 x 3 miles on days 12 and 13. In addition to not running or riding hard for 16 days, when I did start back is was at a decidedly lower level and it has now been another two weeks and my intensity is nowhere near what it was before I got sick. I suspect that it will be a month to get even close to that level (although with winter, it will be a bit reduced), meaning that it dropped me down for two months and I am likely to be at least at a somewhat level level for another month.
Clearly, this malady could have played havoc during XC season. We do not know who got it and when, so do not be too harsh on people that did not perform to the level that you think that they should unless you know the full story.