Usually in people with MS, it's a bladder infection, but not always.
My Neuro does
not consider it a flare up unless a patient has symptoms that last
a least a couple of weeks. Some relapses can go on as long as a month or more. My longest one was about 4 months. A Neuro also would like to see at least a month or more between relapses, before she will say, that you had a remission in between relapses. Hence, relapsing/remitting symptoms.