The symptoms appear in the form of relapses, which are of a variable duration and which vary from one episode to the other, according to the affected area in the CNS. After the flare-ups, symptoms subside and usually disappear completely, although there may be neurological aftereffects. In primary-progressive MS, a progressive clinical deterioration occurs, without relapses, from the onset of the illness. This type of MS is suffered by 12% of people with MS. In secondary-progressive MS, patients initially present a relapsing-remitting course, however, over time, they develop a progressive clinical deterioration without experiencing distinctive relapses. Progressive-recurrent MS, is the least common type of illness.