Despite the ongoing efforts of medical scientists to reveal the factors responsible for causing multiple sclerosis, the exact reasons why the disorder occurs in otherwise healthy persons remain unknown. Although various factors are suspected to trigger the occurrence of multiple sclerosis (viral or bacterial infections) or to facilitate its progression (genetic dysfunctions and various external, environmental factors), medical scientists haven’t yet reached a consensus regarding the exact causes of the disorder.Multiple sclerosis is a type of autoimmune disorder, involving abnormal activity of the immune system, which turns against healthy body cells instead of fighting against intruding infectious agents. The body cells which are primarily targeted by the immune system in the case of patients with multiple sclerosis are nerve cells. In fact, the autoimmune response of the body doesn’t destroy nerve cells directly; the dysfunctional immune system produces antibodies that destroy myelin, a protein that wraps around the cells of the nervous system, facilitating the communication between the central nervous system (CNS – brain and spinal cord) and peripheral nerves and vice versa.At first, multiple sclerosis causes impairments only at the level of the central nervous system. In more advanced stages of progression, multiple sclerosis can involve virtually any innervated region of the body (any part of the body that contains a network of nerves). Without proper medical treatment – which is most effective when administered in early stages of disease – multiple sclerosis can cause a variety of disabilities and sometimes even death.Depending on its patterns of progression, as well as the intensity and frequency of its generated symptoms, multiple sclerosis can be categorized in seven different subtypes. The first subtype of multiple sclerosis is the relapsing-remitting multiple sclerosis (RR MS), the most common form of the autoimmune disorder. According to statistics, more than 80 percent of all multiple sclerosis cases are of the relapsing-remitting subtype. This subtype is characterized by phases of symptomatic remission, followed by phases of relapse (characterized by sudden intensification of symptoms). The duration of the phases of relapse and remission vary from a patient to another, lasting anywhere from several weeks to several years.The second subtype of multiple sclerosis – primary-progressive multiple sclerosis (PP MS) accounts for around 20 percent of all multiple sclerosis cases. The major characteristics of this subtype are gradual progression of the disease, with very short phases of remission. The third multiple sclerosis subtype is similar to the PP MS subtype and is called secondary-progressive multiple sclerosis (SP MS). Patients with primary-progressive multiple sclerosis have 50 percent chances to eventually develop secondary-progressive multiple sclerosis.The fourth subtype of multiple sclerosis is called progressive-relapsing multiple sclerosis (PR MS) and is characterized by gradual progression with frequent phases of symptomatic exacerbation. The fifth multiple sclerosis subtype alternates between the primary-progressive, secondary-progressive and progressive-relapsing forms of the disease. The sixth multiple sclerosis subtype is benign multiple sclerosis, characterized by an initial symptomatic flare which can be followed by slow or no progression at all. The seventh and last multiple sclerosis subtype is also very rare. It is called malignant multiple sclerosis and involves rapid progression and very intense symptoms. This subtype is in most cases deadly.