Although thousands of researchers across the world are trying to solve the puzzle, the exact cause of MS remains a mystery. Current research suggests that several factors may play a role in its development, including genetics and something in a person's environment, possibly a virus.

Genetic Factors

Genetic factors probably play a role in making someone susceptible to MS, but it is currently believed that no single gene is responsible for causing it. This is known as genetic predisposition and is different from a genetic or hereditary illness that is passed directly from parent to child.

Evidence suggests that MS occurs often enough in the same families that it is unlikely to be a coincidence. Take a look at the some of the statistics:

• The risk of developing MS is higher if another family member is affected, so if your brother, sister, parent, or child has MS, you have a 1 to 3 percent chance of developing it yourself.

• An identical twin runs a 30 percent chance of acquiring MS if her twin has the disease, whereas a nonidentical twin has only a 4 percent chance if her twin has it.

• In the United States the prevalence of MS is higher in Caucasians than in other racial groups. Caucasians have twice the incidence rates of African Americans.

• The higher prevalence of MS among people of northern European background suggests some genetic susceptibility. Native Indians of North and South America, the Japanese, and other Asian peoples have very low incidence rates.

These statistics suggest that genetic factors play a role in MS, but other data point to triggers in the environment. Some researchers feel that MS develops because a person is genetically more likely to react to something in the environment. Then when she comes in contact with that agent, it triggers the immune system to attack the CNS. Sophisticated new techniques for identifying genes may help answer questions about the role of genetics in the development of MS.

Environmental Factors

Environmental factors associated with MS only serve to deepen the mystery, as several interesting patterns have been discovered by researchers over the years.

MS is several times more prevalent in temperate climates, so those living farthest away from the equator have a higher risk of developing the disease. A map of the United States shows that the prevalence of MS increases in northern latitudes, where the occurrence of MS is significantly higher than that observed in southern states such as Florida. This might indicate some triggering factor in the environment, such as toxins or vitamin deficiencies, causing MS to manifest in those whose immune systems are genetically predisposed to it.

Studies are now under way to determine if a lack of vitamin D may play a role, as natural sunlight is responsible for the manufacturing of the vitamin in your body. The findings may help explain why both MS and rheumatoid arthritis are more common in northern climates, where sunlight is often scarce.

Migration patterns also seem to play a role in the development of MS. Studies have shown that those who live in areas of the world with a high risk of MS and then move to an area with a low incidence before they turn fifteen years old have a lower risk of developing the disease. This data may suggest that exposure to some unknown environmental agent before puberty may predispose someone to develop MS at a later time.

Scientists periodically receive reports of “MS clusters.” The most infamous case took place in the Faeroe Islands north of Scotland in the years following the arrival of British troops during World War II. Despite years of study of this and other clusters, scientists have not been able to pinpoint a direct environmental factor.

Gender Differences

By most recent studies, it appears that women are two to three times more likely to develop MS than men. The ratio of women to men may be increasing, since studies several decades ago indicated that the ratio of women to men was about two to one.

Scientists are looking for possible reasons for the increase, including lifestyle choices and environmental triggers, but one interesting target of research is hormones. Young women tend to acquire autoimmune diseases at a higher rate than young men — perhaps because men's testosterone levels are high enough to prevent these diseases. Studies have also shown that women have fewer relapses during pregnancy, suggesting estrogen, progesterone, or other pregnancy-related hormones may play a stabilizing role in pregnant women.

Viral Causes

One of the top suspects for triggering MS is a viral agent, and yet, after years of research, one specific virus has not emerged as a proven trigger. Viruses seem like a good candidate because the immune response to viruses may cause demyelinating disease in humans and animals. Proteins of some viruses are similar to those of myelin, and some scientists theorize that this may cause confusion in the immune system, causing the immune cells to attack their own protein as well as that of the virus.

Scientists speculate that an infectious agent almost everyone has come in contact with may cause an abnormal reaction in the immune system in people who are already at risk of developing the disease. Some studies have suggested that many viruses such as measles, herpes, and the flu viruses may be associated with MS. To date, however, this belief has not been proven.

Here are the primary suspects:

• Human Herpesvirus 6 (HHV-6)

  Herpes virus 6 is a common virus that causes a condition in young children known as roseola. Some studies indicate that HHV-6 infection may play a role in MS.

• Epstein-Barr Virus (EBV)

  Evidence suggests an association between EBV, the cause of mononucleosis, and MS. Like HHV-6, EBV is an extremely common virus. Nearly all people have been exposed to EBV. Some researchers propose that people with MS have an atypical immune response to EBV, and this response may increase the risk of MS.

• Chlamydia Pneumoniae

  This atypical bacteria 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. Other viruses that have been investigated include measles virus, adenovirus, and the retroviruses (HIV, HTLV-I, and HTLV-II), but none have emerged as having any definitive importance at this time.