My understanding is that MS is usually defined by the deterioration of the myelin sheath in brain cells which can be detected through MRI's.

A collection of symptoms is a syndrome. Once there is a known definite cause, reclassification as a disease can take place. Lay misuse of the terms and reluctance to adopt updated designations have aided in a loss of distinction in what they refer to.

There is something else entirely to be said for how quickly one can progress from the disbelief of a particular diagnosis as anything beyond a punchline to descanting over the marvels of modern medicine as they relate to futher study of the mechanisms. If nothing else, it's a great reminder of how much information is available to us on a whim.