Multiple sclerosis (MS) is triggered by a combination of genetic susceptibility and environmental exposures, with research identifying several key environmental factors: low vitamin D levels, Epstein-Barr virus infection, smoking, childhood obesity, and geographic latitude. No single factor causes MS on its own, but each one shifts the odds, and several of them interact with each other in ways that help explain why MS rates have been climbing in many parts of the world.
Epstein-Barr Virus
Epstein-Barr virus (EBV), the virus behind mononucleosis, is the strongest known environmental risk factor for MS. A landmark 2022 study tracking over 10 million U.S. military personnel found that EBV infection increased the risk of developing MS by 32-fold. Among people with MS, nearly 100% show evidence of prior EBV infection, compared to roughly 95% of the general adult population. That small gap becomes significant at the population level.
The leading theory is molecular mimicry: a protein on the surface of EBV closely resembles a protein found in the brain and spinal cord. When the immune system learns to attack EBV, it may accidentally begin attacking myelin, the protective coating around nerve fibers. This doesn’t happen in everyone who catches EBV, which is why genetic susceptibility and other environmental factors matter. But EBV appears to be a near-necessary prerequisite. People who never contract EBV almost never develop MS.
The timing of infection may also matter. Getting EBV later in life, particularly during adolescence or early adulthood when it’s more likely to cause mono, seems to carry a higher MS risk than catching it during early childhood, when the infection is typically mild or silent.
Vitamin D and Sunlight Exposure
MS is far more common in regions farther from the equator, where sunlight is weaker and people produce less vitamin D through their skin. Countries like Canada, Scotland, and Scandinavia have some of the highest MS rates in the world, while MS is relatively rare near the equator. This geographic gradient was one of the earliest clues that vitamin D plays a role.
Blood studies back this up. People with low vitamin D levels have a significantly higher risk of developing MS. One large prospective study found that among white adults, those with vitamin D levels in the top 20% had a 62% lower risk of MS compared to those with the lowest levels. The relationship appears strongest during adolescence and early adulthood, suggesting there may be a critical window when vitamin D helps calibrate the immune system.
Vitamin D acts as an immune regulator, helping to suppress the overactive immune responses that drive MS. It influences how certain white blood cells behave and may help maintain tolerance to the body’s own tissues. People who work outdoors, live at lower latitudes, or take vitamin D supplements all tend to have lower MS risk, though clinical trials testing whether vitamin D supplementation can prevent MS in high-risk groups are still underway.
Smoking and Air Pollution
Smoking increases the risk of MS by roughly 50% compared to never smoking, and the risk rises with the number of cigarettes smoked and the years spent smoking. The relationship is dose-dependent: heavier smokers face higher odds. Secondhand smoke exposure during childhood also appears to raise risk, even in people who never smoke themselves.
Once MS develops, continued smoking accelerates the disease. Smokers with MS convert from the relapsing-remitting form to the progressive form faster, accumulate more disability, and show more brain lesion activity on MRI scans. Quitting smoking after diagnosis is associated with a slower rate of progression.
The mechanism likely involves lung irritation triggering immune activation. Inhaled irritants can damage lung tissue in ways that cause immune cells to become more aggressive, and some of those activated cells cross into the central nervous system. This same logic extends to other forms of air pollution. Emerging evidence links exposure to fine particulate matter and organic solvents to elevated MS risk, though the data isn’t as strong as it is for smoking.
Childhood and Adolescent Obesity
Being overweight or obese during childhood and adolescence roughly doubles the risk of developing MS later in life. A large genetic analysis using a method called Mendelian randomization (which uses genetic variants associated with higher BMI to mimic a natural experiment) confirmed that higher BMI in early life has a causal effect on MS risk, not just a correlation.
Excess body fat contributes to a state of chronic low-grade inflammation. Fat tissue produces signaling molecules that push the immune system toward a more inflammatory profile. Obesity also lowers circulating vitamin D levels because vitamin D is fat-soluble and gets sequestered in fat tissue, creating a compounding effect with another major risk factor. The adolescent window appears particularly important, consistent with the pattern seen for vitamin D and EBV timing.
Gut Microbiome
The community of bacteria living in the gut plays a larger role in immune regulation than scientists appreciated even a decade ago. People with MS show measurable differences in their gut microbiome compared to healthy controls, with lower levels of certain bacteria that produce short-chain fatty acids. These fatty acids help maintain immune tolerance and reduce inflammation throughout the body.
Diet, antibiotic use, and early-life microbial exposures all shape the gut microbiome, which makes this factor a potential intersection point for several other environmental risks. A Western diet high in saturated fat and low in fiber promotes a less diverse microbiome, while diets rich in plant foods support the bacterial populations linked to healthy immune function. Research in animal models of MS has shown that transplanting gut bacteria from people with MS into mice can worsen disease, while bacteria from healthy donors can be protective.
Shift Work and Sleep Disruption
Working night shifts, particularly during adolescence, is associated with a higher risk of MS. A Swedish study found that people who worked shifts before age 20 had roughly a 70% higher risk of developing MS compared to those who never did shift work. The effect was smaller but still present for shift work starting at older ages.
Disrupted circadian rhythms alter immune function and lower melatonin production. Melatonin has anti-inflammatory properties and helps regulate the type of immune cells involved in MS. Shift work also reduces sleep quality, which independently affects immune regulation. This factor likely contributes less to overall MS cases than EBV or vitamin D, but it adds to the picture of how modern lifestyle patterns can tip the immune system toward autoimmunity.
How These Factors Interact
The environmental risk factors for MS don’t operate in isolation. They layer on top of each other and interact with genetic predisposition in ways that multiply risk. Someone who carries the HLA-DRB1*15:01 gene variant (the strongest genetic risk factor for MS), had mononucleosis as a teenager, grew up at a high latitude with low vitamin D, and smoked in their twenties faces a dramatically higher risk than any single factor would predict.
Several of these factors cluster together naturally. Living at high latitudes means less sunlight and lower vitamin D. Northern countries tend to have higher rates of EBV infection during adolescence rather than early childhood. Indoor lifestyles reduce sun exposure while increasing the likelihood of obesity. This clustering helps explain why MS rates in some northern European countries exceed 200 cases per 100,000 people, while rates near the equator can be below 5 per 100,000.
The consistency of the adolescent window across multiple risk factors is striking. Vitamin D levels during the teenage years, EBV infection timing, adolescent obesity, and early shift work all point to a period when the immune system is still maturing and particularly susceptible to being pushed toward autoimmunity. Migration studies reinforce this: people who move from a high-risk region to a low-risk region before adolescence take on the lower risk of their new home, while those who move after adolescence carry the higher risk of where they grew up.