A relapse is a sudden onset of new neurological symptoms, or significant worsening of existing MS symptoms. Relapses are also known as “episodes”, “exacerbations” or “flare-ups”. No two relapses are alike, and symptoms vary from person to person and from one relapse to another. Some relapses produce only one symptom (related to inflammation in a single area of the central nervous system) while other relapses cause two or more symptoms at the same time (related to inflammation in more than one area of the central nervous system). They can range from mild to serious in severity. To be medically considered a relapse, it must last at least 24 hours and be separated from the previous relapse by at least 30 days. There should be no other possible cause for the relapse, such as infection, recent surgery or overheating. Most relapses last from a few days to several weeks or even months.