When you have multiple sclerosis (MS), you can be feeling pretty good one day, and the next, notice your symptoms starting to flare. As many as 85% of people with MS have what’s known as relapsing-remitting multiple sclerosis (RRMS), according to the National Multiple Sclerosis Society. This is especially common for people early in the course of the disease.

As its name suggests, RRMS is characterized by going from periods of relapse followed by periods of remission and back again. A relapse is when symptoms worsen (or new symptoms appear), and remission is when symptoms improve, either going away completely or partially.

MS relapses are also called exacerbations, attacks, flares, or flare-ups. Most people find that relapses develop slowly over two to three days and reach a peak after several days. To be considered an MS relapse by the medical community, a flare must: