I'd have to guess it's the location of their lesions. Makes sense to me since some lesions cause no noticeable sx and others cause severe sx.

I know that when I'm stressed out it causes my sx to be magnified, my cognition to become even more impaired and my vision to blur. But.....heat also causes this. Still stress....just in a different form.

Like all the experts agree, MS is different in everyone. My MS and your MS might seem like two different diseases altogether yet we're both dx with MS. I honestly don't think any definite conclusions can be drawn as to what causes MS to flare up......any given day it could be any number of things. Or nothing at all.