Multiple sclerosis (MS) has no single cause. It develops when the immune system attacks the protective coating around nerve fibers in the brain and spinal cord, but what triggers that attack appears to be a combination of genetic susceptibility, viral infection, and environmental factors. Around 2.9 million people worldwide live with MS, and that number has been rising steadily over the past decade.
What Happens Inside the Body
MS is fundamentally an immune system problem. Your nerve fibers are wrapped in an insulating layer called myelin, which helps electrical signals travel quickly and efficiently. In MS, immune cells that normally fight infections cross from the bloodstream into the brain and spinal cord, where they attack myelin as though it were a foreign invader.
Several types of immune cells are involved. One key group consists of helper T cells that react to myelin proteins. In people with MS, these cells are more likely to be in an activated, aggressive state compared to healthy individuals. Some of these cells produce inflammatory molecules that sustain a chain reaction of damage, while others release substances that directly kill the cells responsible for making and maintaining myelin. B cells also play a role, producing antibodies that target structures in the central nervous system and helping to activate T cells.
The result is patches of damage, called lesions, scattered across the brain and spinal cord. Where those lesions form determines which symptoms appear: vision problems, numbness, difficulty walking, fatigue, or cognitive changes. Over time, repeated attacks can lead to permanent nerve damage.
Epstein-Barr Virus Is the Strongest Known Trigger
The most significant breakthrough in understanding MS causes came from a large longitudinal study tracking over 10 million U.S. military personnel. The findings were striking: infection with Epstein-Barr virus (EBV), the virus that causes mononucleosis, increased the risk of developing MS 32-fold. No other virus produced a similar effect.
EBV infects roughly 95% of adults worldwide, yet only a small fraction develop MS. This means the virus is likely necessary but not sufficient on its own. The leading theory is that EBV infection primes the immune system to mistakenly recognize myelin as a threat, but this only leads to MS in people who carry other risk factors. The timing of infection may also matter. People who contract EBV later in adolescence or adulthood, when it’s more likely to cause full-blown mono, appear to face higher risk than those infected in early childhood.
Genetic Susceptibility
MS is not directly inherited, but genetics load the dice. More than 100 genetic variants have been identified that increase susceptibility, most of them involved in immune system regulation. The single most influential gene variant sits in the HLA region of chromosome 6, which helps the immune system distinguish the body’s own tissues from foreign invaders. Carrying the HLA-DRB1*15:01 variant roughly triples the risk of developing MS.
Having a first-degree relative with MS raises your risk to about 2-4%, compared to roughly 0.1% in the general population. Studies of identical twins show that when one twin has MS, the other develops it only about 25-30% of the time. That gap tells us genetics alone can’t explain the disease. Something in the environment has to pull the trigger.
Vitamin D and the Latitude Effect
MS becomes more common the farther you live from the equator. A meta-analysis of global data found that for every degree of latitude away from the equator, MS prevalence increases by roughly 4 to 5 cases per 100,000 people. Scandinavia, northern Europe, Canada, and the northern United States have some of the highest rates in the world. This gradient has been growing stronger over time, suggesting that modifiable environmental factors like sun exposure remain a major driver of risk.
The most likely explanation is vitamin D, which the body produces in response to sunlight. A study from Harvard Medical School found that people with vitamin D blood levels in the top 20% (above roughly 100 nmol/L) had a 62% lower risk of MS compared to those in the bottom 20% (below about 63 nmol/L). Vitamin D plays a direct role in regulating immune function, and low levels may make the immune system more prone to the kind of misdirected attacks that characterize MS.
People who migrate from high-risk to low-risk regions before adolescence tend to adopt the lower risk of their new home, while those who move after puberty carry their original risk with them. This points to a critical window during childhood and adolescence when environmental exposures shape long-term MS risk.
Smoking and Disease Progression
Smoking increases MS risk by about 50% compared to never smoking. But the effects don’t stop at diagnosis. Smokers with relapsing-remitting MS are significantly more likely to progress to the secondary progressive form of the disease, where disability accumulates steadily rather than in distinct relapses. One study found that current smokers progressed to this stage 2.5 times faster than people who had never smoked. Brain scans tell the same story: smokers accumulate more lesions and lose brain volume faster than nonsmokers with MS.
The mechanism likely involves both direct toxic effects on the central nervous system and further stimulation of the already overactive immune response. Among all modifiable risk factors, smoking is one of the most actionable, since quitting has been shown to slow disease progression.
Childhood Obesity
Being overweight during childhood and adolescence is an emerging risk factor. A large cohort study found that children with a BMI at or above the 95th percentile faced roughly 1.5 to 2 times the risk of developing MS later in life compared to children with a BMI below the 85th percentile. The association was strongest in girls between ages 7 and 13, where each standard unit increase in BMI raised risk by 17-21%. Boys showed a similar but narrower window of vulnerability, around ages 8 to 10.
Fat tissue is metabolically active and produces inflammatory molecules. Excess body fat during adolescence may create a low-grade inflammatory state that, combined with other risk factors like EBV infection or low vitamin D, tips the immune system toward autoimmunity. Obesity is also associated with lower vitamin D levels, since the vitamin gets sequestered in fat tissue, so the two risk factors may reinforce each other.
Gut Bacteria and Immune Balance
The community of bacteria living in your digestive tract plays a surprisingly important role in training and regulating the immune system. People with MS tend to have a different bacterial profile than healthy individuals. Certain beneficial bacteria, particularly from the Bacteroides, Prevotella, and Lactobacillus groups, are found in lower amounts. Meanwhile, other species, notably Akkermansia muciniphila, tend to be overrepresented.
A study comparing identical twins where only one had MS found that the twin with MS had higher levels of Akkermansia in their gut, even when not on treatment. This is notable because identical twins share virtually all their DNA, suggesting the difference in gut bacteria isn’t just genetic. Whether these bacterial shifts are a cause or consequence of MS remains an active question, but animal studies show that transplanting gut bacteria from MS patients into healthy mice can trigger MS-like immune responses, pointing toward a causal role.
How These Factors Work Together
No single cause explains MS. The most widely accepted model is that the disease requires multiple hits. A person inherits genetic variants that make their immune system prone to attacking myelin. During childhood or adolescence, environmental factors like low vitamin D, obesity, or limited sun exposure further shape that vulnerability. Then EBV infection activates the immune system in a way that breaks tolerance to myelin. Smoking, gut bacteria imbalances, or other inflammatory exposures can amplify the process.
This layered model explains why MS clusters in certain populations and geographies without following simple inheritance patterns. It also explains why the disease is roughly two to three times more common in women, who tend to have more reactive immune systems. Most importantly, it means that several of the contributing factors, including vitamin D status, smoking, and childhood weight, are potentially modifiable, even if the genetic and viral components are not.