Well, your choice, of course, as they have no right to know details, but you could say: “I’m on the sick as I was diagnosed with a serious illness”, or even tell them outright it’s MS, as “on the sick” doesn’t really convey much info.
Unfortunately, it’s a common problem with all invisible illnesses.
My dad was diagnosed with terminal cancer, and for a short time afterwards (not very long at all), he was still out and about, walking three miles, and doing 40 lengths at the pool! Well-meaning people greeted him in the street with: “It’s so great to see you’re on the mend!”
“I’m not on the mend.”
“What? How do you mean?”
“I’m NOT on the mend - there’s nothing they can do.”
I used to feel awful for both the enquirer and Dad, because they’d obviously realise they’d put their foot in it big time, but he, on the other hand, was forced to explain every time that his illness wasn’t survivable - which is hard for people to comprehend, when they see the person out and about, looking normal.