Causes

Infectious Organisms Under Suspicion. Although many infectious microorganisms have been investigated, no one organism has emerged as a proven trigger. It is possible different patients may be affected by different organisms, and that infections cause some, but not all, cases of MS. Organisms that are at the top of the suspect list are those that can affect the central nervous system. The following are three primary suspects:

• HHV-6. Herpesvirus 6 (a form of herpesvirus that causes roseola, a benign disease in children) is also known to cause encephalitis (brain inflammation) in patients with impaired immune systems. A number of studies have reported higher than normal rates of HHV-6 infection in patients, and some experts believe that may be important in MS. Other experts argue, however, that nearly everyone harbors this virus and there is still no evidence of a causal relationship. Other herpes viruses can also infect brain cells. They include herpes simplex 1 and 2 (the causes of oral and genital herpes), varicella-zoster virus (the cause of chicken pox and shingles), and cytomegalovirus.
• Epstein-Barr virus (EBV). Evidence suggests an association between EBV, the cause of mononucleosis, and MS. EBV is an extremely common virus and another member of the herpes virus family. Nearly all people have been exposed to EBV. However, researchers have discovered that people who are especially sensitive to the virus and have unusually high levels of EBV antibodies may have a greater risk of developing MS. Scientists are still uncertain if EBV is a cause of MS. EBV has also been linked to other autoimmune diseases such as lupus.
• Chlamydia Pneumoniae. This atypical bacterium has been associated with persistent inflammation. A few studies have reported significantly higher rates of previous Chlamydia infection in patients with MS than in individuals without MS. An important group of 2000 studies reported no connection at all between Chlamydia and MS, and any experts now believe there is no strong evidence linking the microbe to MS. It is still possible, however, that the infection, which can cause widespread inflammation, plays a role early in the course of the disease in some individuals.