Multiple sclerosis (MS) isn’t caused by a single thing. It develops when a combination of genetic susceptibility, viral exposure, and environmental factors converge to trigger an immune system attack on the protective coating around nerve fibers in the brain and spinal cord. No one “catches” MS, and no single behavior guarantees you’ll develop it. But researchers have identified a clear set of factors that raise or lower your risk, and many of them interact with each other in ways that matter.
What Actually Happens in the Body
MS is an autoimmune disease. Your immune system, which normally fights infections, mistakenly targets myelin, the insulating layer that wraps around nerve fibers in your brain and spinal cord. Myelin works like the coating on an electrical wire: it helps signals travel quickly and smoothly. When immune cells damage it, those signals slow down, get distorted, or stop entirely.
The process starts when certain immune cells cross the blood-brain barrier, a tightly sealed boundary that normally keeps most immune activity out of the central nervous system. In the early stages of MS, inflammatory damage to this barrier allows immune cells to flood into the brain and spinal cord, where they attack myelin and eventually the nerve fibers underneath. The location and severity of these attacks determine your symptoms, which is why MS looks so different from person to person. One person might have vision problems, another might have numbness in their legs, and someone else might struggle with balance or fatigue.
Genetics Set the Stage
MS is not directly inherited, but your genes play a significant role in how vulnerable you are. The strongest known genetic link involves a specific immune system gene called HLA-DRB1*15:01, which increases MS risk in white populations by about threefold. This gene helps your immune system distinguish between your own cells and foreign invaders. When it functions differently, the immune system may be more prone to misidentifying myelin as a threat.
More than 200 other gene variants have been linked to MS, each contributing a small amount of additional risk. Having a first-degree relative (parent, sibling) with MS raises your lifetime risk to roughly 2 to 4 percent, compared to about 0.1 percent in the general population. Identical twins share MS only about 25 to 30 percent of the time, which tells us genes alone aren’t enough. Something in the environment has to pull the trigger.
The Epstein-Barr Virus Connection
Of all the environmental factors studied, infection with Epstein-Barr virus (EBV) has the strongest and most consistent link to MS. EBV is the virus that causes mono (infectious mononucleosis), and it infects roughly 95 percent of adults worldwide. Most people carry the virus without problems, but something about the infection appears to set the stage for MS in genetically susceptible individuals.
A population-based study found that people who developed symptomatic mono from EBV had a more than threefold increase in MS risk, with an adjusted hazard ratio of 3.14. A landmark 2022 study tracking 10 million U.S. military personnel found that MS risk increased 32-fold after EBV infection, and virtually no cases of MS occurred in people who had never been infected. The leading theory is that EBV trains certain immune cells to react to a viral protein that closely resembles a component of myelin, a case of molecular mistaken identity.
Vitamin D and Where You Live
MS is far more common in regions farther from the equator, where sunlight is weaker and people produce less vitamin D. This geographic pattern is one of the most striking features of the disease. Countries like Canada, Scotland, and Scandinavia have significantly higher rates than countries near the equator.
Vitamin D appears to play a direct role in regulating immune function. A Harvard Medical School study found that white individuals with vitamin D blood levels in the top 20 percent (above 99.1 nmol/L) had a 62 percent lower risk of developing MS compared to those in the bottom 20 percent (below 63.3 nmol/L). That’s a substantial difference. People who spend more time outdoors, live in sunnier climates, or have higher dietary vitamin D intake consistently show lower MS rates. Moving from a high-risk region to a low-risk one before adolescence also appears to reduce risk, suggesting that vitamin D exposure during childhood and teenage years is particularly important.
Smoking Raises Risk Significantly
Smoking is the best-established modifiable risk factor for MS. People who have ever smoked regularly face roughly a 40 to 50 percent higher risk of developing MS compared to people who have never smoked. The risk increases with the amount and duration of smoking, and it doesn’t fully disappear after quitting, though quitting does help.
Cigarette smoke appears to cause harm in multiple ways. It triggers inflammation in the lungs that can activate immune cells, and the chemicals in smoke may directly damage the blood-brain barrier, making it easier for those activated immune cells to reach the central nervous system. Smoking also interacts with genetic risk factors: people who carry the HLA-DRB1*15:01 gene and smoke have a much higher risk than either factor alone would predict. For people already diagnosed with MS, continued smoking is associated with faster disease progression and more brain damage over time.
Obesity During Adolescence
Carrying excess weight during childhood and early adulthood is linked to higher MS risk, particularly in women. A Kaiser Permanente study found that women with a BMI of 30 or higher in their twenties had roughly double the risk of developing MS compared to women at a healthy weight. The relationship followed a linear pattern: the higher the BMI, the greater the risk.
Being overweight at ages 10 and 20 was also significantly associated with MS in females. The connection likely involves the inflammatory effects of excess fat tissue, which produces signaling molecules that keep the immune system in a heightened state. Adolescence appears to be a critical window, possibly because the immune system is still maturing and more susceptible to being pushed toward autoimmune patterns.
Chemical Exposures and Occupational Risks
Prolonged exposure to organic solvents, the chemicals found in paints, varnishes, degreasers, and adhesives, has been linked to roughly a twofold increase in MS risk. An early meta-analysis of thirteen studies found an association in ten of them. One of the first studies on this topic, looking at shoemakers in Florence, Italy who were heavily exposed to solvents, found an odds ratio close to five.
The risk from solvent exposure appears to be amplified when combined with smoking. People who both smoke and have occupational solvent exposure face a significantly higher risk than either factor alone would suggest, following the same pattern of gene-environment interaction seen with other MS risk factors. This means that for people who work around industrial chemicals, avoiding cigarettes becomes even more important.
Who Gets MS Most Often
MS affects women far more than men. Globally, about 74 percent of people living with MS are female, giving a roughly 3-to-1 female-to-male ratio. This gap has widened over the past several decades, and researchers suspect hormonal differences, particularly estrogen’s effects on the immune system, play a role. MS is most commonly diagnosed between ages 20 and 40, though it can appear earlier or later.
Ethnicity also matters. MS is most common in people of Northern European descent and less common in people of East Asian, African, and Indigenous ancestry, though it occurs in every ethnic group. Some of this variation maps onto the HLA gene differences between populations, but it also reflects differences in sun exposure, diet, and other environmental factors.
Gut Bacteria May Play a Role
A growing body of evidence connects the gut microbiome to MS risk. A Yale School of Medicine study found that people with MS have measurably different gut bacteria compared to healthy individuals. Specifically, those with MS had lower levels of certain beneficial bacteria, including species of Faecalibacterium, which are known for their anti-inflammatory effects. They also had higher levels of some potentially harmful species, like a Monoglobus strain that was more abundant in untreated patients.
Perhaps most intriguingly, people recently diagnosed with MS had a significantly smaller proportion of gut bacteria coated in a protective antibody called IgA, which helps the immune system keep gut bacteria in check. This suggests the immune system’s relationship with gut microbes is disrupted early in the disease. Whether these gut changes are a cause or consequence of MS is still being worked out, but the connection is consistent enough that it likely plays a meaningful role in how the disease develops.
Why It Takes Multiple Factors
The clearest takeaway from MS research is that no single factor is sufficient on its own. You need genetic vulnerability, likely involving dozens of genes. You almost certainly need EBV infection at some point. Then additional factors like low vitamin D, smoking, obesity, or chemical exposures pile on to tip the balance. Many of these risk factors interact with each other multiplicatively rather than just adding up, which explains why MS clusters in certain populations and geographic regions but remains relatively unpredictable at the individual level.
This also explains why MS can seem to appear out of nowhere. Someone might carry genetic risk their entire life, get infected with EBV as a teenager, and not develop symptoms until their thirties, when some additional stressor or accumulated damage finally pushes the immune system past its threshold. The disease likely begins years before the first noticeable symptoms, with small areas of immune activity in the brain building gradually until they cause problems you can feel.