MS causes and risk factors
Fact-checked by The exact causes of multiple sclerosis (MS) are not fully understood. Scientists know that MS develops when the immune system mistakenly attacks the brain and spinal cord, but they do not yet know what initially triggers this process.
Still, MS is believed to arise from a combination of factors. While no single factor causes MS on its own, a range of genetic, infectious, environmental, and demographic factors can increase a person’s likelihood of developing the disease.
Understanding what causes MS — and why some people develop it while others do not — may help researchers improve prevention strategies and develop more effective MS treatments.
What causes MS?
The immune system defends the body against foreign invaders, such as viruses and bacteria. Put simply, its job is to attack anything that could cause injury or disease, while leaving healthy tissue alone.
In autoimmune diseases, however, the immune system mistakenly recognizes certain cells or tissues in the body as a threat and launches an inflammatory attack against them.
In all types of MS, these immune attacks target the myelin sheath, a fatty substance that wraps around nerve fibers and helps them send electrical signals quickly and efficiently.
Damage to myelin causes inflammation and tissue damage that:
- strip myelin from nerve cells and damage the nerves themselves
- trigger repeated but often incomplete repair attempts, resulting in scar-like lesions in the brain and spinal cord
- disrupt nerve cell signaling, causing a wide range of MS symptoms
Scientists still don’t fully understand why the immune system begins attacking myelin in MS. Most evidence suggests that the disease develops when multiple factors — including genetics and environmental exposures — interact over time to cause immune dysfunction.
Genetic factors
MS is not a heritable disease, meaning that it is not directly passed from parents to their biological children. Nonetheless, a person’s genetic makeup is known to affect MS risk, and about 1 in 5 patients have a family history of the disease. In addition:
- People with a first-degree relative (parent, sibling, or child) who has MS have about a 2%-3% risk of developing it over their lifetime.
- Identical twins share the same genetic makeup, yet if one twin has MS, the other develops it only about 25%–30% of the time.
- People adopted into families affected by MS are not at increased risk, supporting the idea that genetics has a stronger influence on MS risk than a shared environment.
More than 200 genetic variants have been identified as possibly contributing to MS development. The strongest genetic risk factor for MS is a particular variant of the HLA-DRB1 gene, called HLA-DRB1*15:01. This gene helps the immune system distinguish between harmful invaders and the body’s own healthy tissues.
Infections and viruses
Certain infections have been linked to an increased risk of developing MS, suggesting that immune responses to viruses may play a role in triggering the disease in susceptible individuals.
Infection with the Epstein-Barr virus (EBV) has emerged as one of the strongest known MS risk factors, raising the risk of developing the autoimmune disorder by more than 30 times. While EBV infects most people at some point in their lives, the vast majority will never know they’ve had it.
Scientists are still working to better understand the link between EBV and MS. One leading theory is that structural similarities between certain EBV and myelin proteins may confuse the immune system, causing it to mistakenly attack the nervous system when attempting to control the virus.
Importantly, EBV infection alone is not enough to cause MS — it likely interacts with other risk factors to trigger the disease — but researchers now believe that it is necessary for MS to develop.
Other infections that have been linked to MS include:
- herpesviruses, particularly human herpes virus 6
- human endogenous retroviruses, remnants of ancient viral DNA embedded in the human genome
The exact role of these infections remains unclear, and research is ongoing to determine whether they contribute directly to MS development.
Autoimmune diseases
MS is an autoimmune disease, meaning it occurs when the immune system mistakenly attacks healthy tissue. Many autoimmune conditions share common risk factors, such as genetics or specific environmental exposures, suggesting they may have common disease-driving mechanisms.
While specific associations and mechanisms are still being investigated, people with certain autoimmune diseases are more likely to develop MS. Conditions that may serve as autoimmune risk factors for MS include:
| Type of autoimmune disease | Potential association |
|---|---|
| Type 1 diabetes | This condition affects insulin production in the pancreas. It has been associated with an up to 20 times higher likelihood of developing MS. |
| Inflammatory bowel diseases | Autoimmune conditions affecting the digestive system, including Crohn’s disease and ulcerative colitis, have been linked to a more than 50% greater chance of developing MS. |
| Thyroid disorders | Graves’ disease and Hashimoto’s disease, conditions that respectively result in high and low levels of thyroid hormones, have been reported more frequently in people with MS than in the general population. |
| Psoriasis | Some studies suggest an association between MS and this autoimmune condition, which causes itchy, scaly skin. |
MS risk factors: Environmental and lifestyle
A number of environmental and lifestyle factors have been linked to an increased risk of developing MS. Although the mechanisms underlying these relationships are not completely understood, they may influence immune system function and interact with genetic susceptibility to increase MS risk.
Environmental risk factors for MS include:
- Geography: MS is generally more common in regions farther from the equator and less common in areas with greater year-round sunlight. Differences in sun exposure and vitamin D levels, as well as genetic and lifestyle factors, likely contribute to these geographic variations in MS risk.
- Vitamin D: This vitamin is produced in the skin in response to sunlight. Low vitamin D levels have consistently been associated with a higher risk of MS, possibly due to its effects on immune cell activity.
- Smoking: People who have ever smoked cigarettes have a roughly 50% higher likelihood of developing MS than those who never smoked. Passive smokers — people who breathe in others’ tobacco smoke — may also be at an increased risk. Research suggests that smoking may create an inflammatory environment that promotes MS onset and progression.
- Obesity: Obesity, particularly during adolescence and early adulthood, has been linked to an increased risk of MS. Excess body fat can promote ongoing inflammation in the body, which may set the stage for MS to develop.
Demographic factors
Certain demographic factors can also influence the risk of developing MS, including:
- Age: MS can develop at any age, but symptoms most often begin in early adulthood. The majority of MS patients are diagnosed between the ages of 20 and 40.
- Sex: MS is about three times more common in women than in men. This disparity is thought to be driven mainly by sex-related hormones, which influence immune and nervous system activity. Rates of MS are similar between men and women before puberty, and the sex gap also narrows after menopause.
- Race and ethnicity: Although MS can affect people of all racial and ethnic backgrounds, race and ethnicity are considered MS risk factors. In the U.S., MS prevalence is highest among non-Hispanic white and Black populations and lowest in Hispanic, Asian, and Pacific Islander populations. These differences could be influenced by genetics, climate, and socioeconomic disparities.
What are not MS risk factors?
Some factors have been shown to have no connection with the risk of developing MS, although in certain cases they may trigger new disease activity in a person who has already been diagnosed.
- Alcohol: There is no evidence that drinking alcohol in moderation increases MS risk. However, heavy drinking after an MS diagnosis may worsen certain MS symptoms or interfere with overall health.
- Routine vaccinations: Scientific evidence does not support a link between common vaccines, including those for COVID-19 or the flu, and an increased risk of MS. In people with the disease, vaccination may help prevent infections that can trigger MS relapses.
- Pregnancy and breastfeeding: While pregnancy can temporarily affect MS disease activity, having children or breastfeeding does not increase the likelihood of developing MS. Some studies suggest these factors may even have a protective effect, although this is not firmly established.
- Other diseases: Certain conditions, including the bacterial infection Lyme disease and the autoimmune disorder Guillain-Barré syndrome, can cause symptoms that resemble MS. However, these are distinct disorders and do not increase the risk of developing MS.
Potential MS risk factors for which more research is needed
For other potential risk factors, current research is contradictory about whether there is a true association with MS. It is possible that an association will be confirmed or excluded with ongoing research.
- Vitamin B12: Also called cobalamin, this vitamin helps maintain healthy myelin, and its deficiency can lead to MS-like symptoms. Some studies have linked B12 deficiency to MS, but others have found no connection.
- Injuries to the head and spine: Studies have linked traumatic injuries — such as concussions from contact sports or whiplash from a car accident — to the development of MS, especially in people with genetic susceptibility. However, evidence remains limited and inconclusive.
- Early life stress or trauma: Some studies have reported associations between childhood trauma and stress with an increased risk of MS, but findings have been inconsistent. More research is needed to clarify the relationship between early life stress and MS.
- Exposure to toxins: Various studies have suggested that exposure to air pollution, organic solvents, or other chemicals could increase MS risk. Still, research in this area is ongoing.
- Dietary factors: Emerging evidence suggests that diets high in saturated fats, salt, and added sugars may promote inflammation, while anti-inflammatory dietary patterns may be protective.
Continued research into what triggers MS and why some people get MS may help scientists develop better strategies for prevention, earlier diagnosis, and more effective treatments in the future.
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