Seth Roberts posted there's a plausible theory that MS is caused by malformed or blocked veins which should be letting blood flow away from the brain.
One possible sign of bad outflow could be having one's face turn a good bit redder than one's body during exercise.
At an MS site, there's a survey about whether you've got MS and whether you get "red head".
Almost all of their responses are from people who have MS, so it isn't clear what proportion of the general population gets that red-faced.
So it would be a good thing whether you have MS or not to go over to their site and answer the survey.