Multiple sclerosis (MS) is caused by the immune system mistakenly attacking the protective coating around nerve fibers in the brain and spinal cord. No single cause has been identified. Instead, MS appears to result from a combination of genetic susceptibility, viral exposure, and environmental triggers that together push the immune system toward this self-destructive behavior. About 85% of cases first appear between ages 18 and 50, and the disease affects women two to three times more often than men.
How the Immune System Attacks the Brain
Nerve fibers in your brain and spinal cord are wrapped in a fatty insulation called myelin, which helps electrical signals travel quickly between neurons. In MS, immune cells that normally fight infections cross the blood-brain barrier and attack this myelin instead. The inflammation clusters around small veins in the brain’s white matter, gradually stripping away the insulation and leaving behind scar tissue (the “sclerosis” in the disease’s name).
The process likely starts outside the brain entirely. Everyone carries immune cells in their blood that can react to myelin proteins. Normally these cells stay dormant. But if they become activated, possibly by an infection whose proteins resemble myelin, they can slip into the central nervous system and trigger inflammation. This concept is called molecular mimicry: the immune system mistakes part of your own nerve coating for a foreign invader because the two look similar at a molecular level.
Once inside the brain, several types of immune cells join the attack. The resulting damage disrupts nerve signaling, which is why MS symptoms can affect vision, movement, balance, and sensation depending on where the scarring occurs.
Genetic Risk Factors
MS is not directly inherited, but your genes significantly influence your likelihood of developing it. The strongest known genetic risk factor is a specific immune system gene called HLA-DRB1*15:01. Carrying one copy of this gene variant raises MS risk roughly 3.7 times compared to people without it. Carrying two copies increases the risk about 7.8 times. This gene helps your immune system distinguish foreign proteins from your own, so variants in it may make the immune system more prone to misidentifying myelin as a threat.
Beyond this single gene, researchers have identified over 200 smaller genetic variants that each contribute a modest amount of risk. Most of these are involved in immune regulation. Having a first-degree relative with MS (a parent or sibling) raises your own risk noticeably compared to the general population, but the majority of people with a family member who has MS never develop it themselves. Genes set the stage, but something environmental has to pull the trigger.
The Epstein-Barr Virus Connection
Of all the environmental triggers studied, the Epstein-Barr virus (EBV) has the strongest link to MS. EBV is the virus that causes mono (infectious mononucleosis) and infects roughly 95% of adults worldwide. Most people carry it without problems, but developing symptomatic mono is associated with a more than threefold increased risk of MS later in life.
The connection is striking because MS is exceptionally rare in people who have never been infected with EBV. Researchers believe the virus may activate those dormant myelin-reactive immune cells through molecular mimicry: certain EBV proteins closely resemble myelin proteins, and the immune response mounted against the virus may accidentally train the body to attack its own nerve insulation. EBV also infects a type of immune cell (B cells) that plays a central role in MS inflammation, potentially keeping the immune system in a state of chronic activation.
Vitamin D and Sunlight Exposure
MS is more common the farther you live from the equator, and vitamin D appears to be a major reason why. A large meta-analysis found that people with vitamin D deficiency, defined as blood levels below 50 nmol/L (about 20 ng/mL), had a 54% higher risk of developing MS compared to those with sufficient levels.
Vitamin D plays an active role in regulating the immune system, helping to suppress the kind of overactive immune responses that drive MS. People living at higher latitudes get less UVB sunlight, especially during winter months, which means their skin produces less vitamin D. This geographic pattern closely mirrors MS prevalence worldwide. The connection also helps explain why MS is diagnosed more frequently in northern Europe, Canada, and the northern United States than in tropical regions.
Smoking and Disease Progression
Cigarette smoking increases MS risk by an estimated 40% to 80% compared to never smoking. The risk appears to rise with the number of years a person smokes. But smoking doesn’t just raise the chances of getting MS. It also makes the disease worse once it starts.
People with MS who smoke progress to disability milestones faster than non-smokers. They are more likely to convert from an initial episode of neurological symptoms to a confirmed MS diagnosis, and they accumulate physical disability more rapidly over time. The mechanism likely involves the lung irritation and chronic low-grade inflammation that smoking produces, which can activate immune cells and push them toward attacking the central nervous system. Quitting smoking is one of the few modifiable factors that can influence both MS risk and its course.
Childhood Obesity
Being overweight during childhood or adolescence is an independent risk factor for MS later in life. A Danish study found that girls with a BMI at or above the 95th percentile had a 1.75-fold higher risk of developing MS compared to girls with a BMI below the 85th percentile. Among severely obese children, the risk was even more pronounced: more than three times the likelihood of MS compared to children at a normal weight, with the effect stronger in girls.
The proposed link is chronic inflammation. Excess body fat, particularly in young people, raises blood levels of inflammatory markers that keep the immune system in a heightened state. Over time, this pro-inflammatory environment may lower the threshold for autoimmune diseases like MS to take hold, especially in someone who already carries genetic risk factors.
Sex and Hormones
The fact that MS affects women two to three times more often than men points to a role for sex hormones and sex-linked immune differences. This gender gap appears to be widening over recent decades, which suggests environmental or lifestyle factors specific to women may be amplifying the effect. The speed of the change rules out genetics alone, since gene pools don’t shift that quickly.
Women generally mount stronger immune responses than men, which protects them better against infections but also makes them more susceptible to autoimmune conditions across the board. Hormonal fluctuations, particularly involving estrogen, may influence when and how aggressively the disease manifests. MS symptoms often improve during pregnancy, when estrogen levels are high, and can flare after delivery when those levels drop.
Gut Bacteria and Immune Balance
Your gut hosts trillions of bacteria that help train and regulate your immune system, and people with MS have a distinctly different gut microbiome compared to healthy individuals. Research published in the Proceedings of the National Academy of Sciences found that people with MS are enriched in certain bacterial species while depleted in others. Specifically, MS patients had lower levels of bacteria that support gut barrier health and higher levels of bacteria that promote inflammation.
One key finding is the ratio between two bacterial groups: Bifidobacterium and Akkermansia. A lower ratio of these bacteria was found in MS patients and correlated with greater disability. Bifidobacterium species help build and maintain the protective mucus lining of the gut, while Akkermansia species feed on that mucus. When the balance tips toward too much mucus consumption and not enough mucus building, the gut barrier weakens and inflammatory signals increase. In animal studies, introducing MS-associated bacteria into mice raised levels of inflammatory markers and worsened disease severity.
This research is still being mapped out in detail, but it suggests that gut health is not just a bystander in MS. The bacterial ecosystem in your intestines may actively influence whether your immune system stays balanced or tips toward the kind of overactivation that damages myelin.
How These Causes Work Together
No single factor causes MS on its own. The current understanding is a layered model: genetic susceptibility creates the foundation, then environmental exposures like EBV infection, low vitamin D, smoking, or childhood obesity stack on top. At some point, the accumulated risk crosses a threshold where the immune system begins attacking myelin.
This explains why identical twins, who share the same genes, have only about a 25% to 30% chance of both developing MS if one is affected. It also explains why MS rates are rising in some populations faster than genetics could account for, pointing to changing environmental and lifestyle factors. The interaction between genes and environment is direct in some cases: carrying the HLA-DRB1*15:01 gene variant while also smoking, for example, multiplies the risk beyond what either factor contributes alone.