Not every flare-up symptoms is necessarily a relapse. You can have what is known as a pseudo-exacerbation, or pseudo-relapse, that’s caused by something like stress or an infection.
There are lots of anecdotal reports, but very little scientific evidence that stress causes relapses, BUT, it’s widely accepted it does aggravate symptoms, and most of us will have experienced that.
I often relate the time when even an over-pushy friend trying to persuade me to go round for dinner when I didn’t want to made me go weak at the knees. That was not a relapse, because it only lasted as long as the phone call, and a little after, but I could feel myself getting weaker as he wouldn’t take no for an answer.
I’m not sure it would happen today, but this was not long after a real relapse, when my lower body strength had been affected. I was recovering well, but lost strength again temporarily every time I got stressed.
Similarly with infections. Unlike stress, there is pretty good evidence they can cause relapses - especially respiratory tract infections, for some reason (that’s why it’s recommended to get the flu’ jab).
But infection-linked relapses are not instantaneous - they could be up to five or six weeks later - i.e. a reaction to the immune system being activated.
But infections can cause a pseudo-relapse, too. This happens while you’ve still actually got the illness. Water infections (UTIs) are well known for aggravating symptoms, and anything that causes a temperature is likely to do so as well - so even a feverish cold. The body’s response to infection is often to raise the temperature (make the environment a inhospitable host), but for those whose MS symptoms are sensitive to temperature, this has predictable results.
Before I was diagnosed, I spent years wondering why I felt like I was dying with a simple cold. I get the same every month with my period, as well - rise in body temperature again - but NOT a real relapse. That’s been helped a lot by going on the pill. I’ve got the OK to run two packets back-to-back without a break, so I only get a period once every two months (naturally it was down to three weeks), and it’s much easier and lighter. Still get a small flare in symptoms, but nothing like it was.
Oh, I did try - three times - to answer your other post about early symptoms by the way, but for some reason the forum wouldn’t have it. I will try again when I’m feeling more patient. It was very hard to explain anyway, because a lot of my early symptoms were very vague and non-specific, and I didn’t really begin to see them as symptoms 'til after I was diagnosed (like the feeling half-dead every time I had a cold or a period, for example).