Quite a few forms of MS exist, which can create difficulty in deciding how to manage the illness. The disease damages myelin. Sometimes, the body can do limited repairs to myelin. When this happens, symptoms (the problems caused by the disease) go away for a short time. This is called remission. When the body attacks the myelin again, the symptoms return and this is called relapse. The type of MS that has remissions and relapses is called Remitting-Relapsing MS. In less common cases, the body continues attacking myelin and the symptoms get worse rapidly, a form called "Primary Progressive MS". Sometimes a combination of the two types of disease can occur simultaneously.