MS isn't, but yes, Epstein-Barr virus is more serious if you first catch it as an adult. That will likely knock you out with mononucleosis, a.k.a kissing sickness, for a few weeks. Fatigue can linger a lot longer.
Definitely my experience. I had it as an adult at 26 and it absolutely knocked me out for weeks. I lost a ton of weight from having no appetite and having a constant sore throat. Sleeping was also terrible because I couldn't breathe easily. I think I missed 3 or 4 weeks of work and even when I started back I could only work half days for awhile at my not-strenuous desk job.
