Multiple sclerosis, often simply called MS, is a disease in which the body’s own immune system mistakenly attacks myelin - the insulating layer that surrounds the nerve fibers in the brain and spinal cord. Normally, the immune system protects the body from viruses and bacteria, but in MS it becomes overactive and attacks healthy cells in the central nervous system. The central nervous system consists of the brain, spinal cord, and optic nerves, which together send information to and from all parts of the body.

When the myelin becomes damaged, inflammatory-like changes occur, called lesions or plaques. This can cause symptoms such as visual disturbances, sensory changes, paralysis in a limb, or problems with balance.

Each time new plaques appear, previous symptoms may flare up again. However, the course of the disease varies greatly from person to person, so the extent to which it affects daily life and functional ability differs widely.

There are three main types of multiple sclerosis:

Relapsing-remitting multiple sclerosis is the most common form. The disease appears in episodes - called relapses - where symptoms flare up for a period and then disappear completely or partially again. About 70% of those with relapsing-remitting MS in Denmark are women. The disease most often appears in the 20s, 30s, or 40s, and the average age at diagnosis is 37 years.

Secondary progressive multiple sclerosis usually develops after several years of relapsing-remitting MS. Over time, the relapses stop, and the disease instead progresses gradually with a slow, continuous worsening of symptoms.

Primary progressive multiple sclerosis affects about 10–15% of people with MS. This form develops slowly and steadily from the start, without clear relapses. Many people with this form experience a gradual decline in functional ability - physically, cognitively, or both. Primary progressive MS usually begins later in life than the other types.