Multiple sclerosis (MS) develops when the immune system mistakenly attacks the protective coating around nerve fibers in the brain and spinal cord. No single cause explains why this happens, but research points to a combination of genetic susceptibility, viral infections, and environmental exposures that together push the immune system toward attacking the body’s own tissue. About 85% of cases are first diagnosed between ages 18 and 50, and women make up roughly 74% of people with MS worldwide.
Genetics Set the Stage but Don’t Seal the Deal
MS is not directly inherited, but your genes influence how likely you are to develop it. The strongest known genetic risk factor is a specific immune system gene variant called HLA-DRB1*15:01. Carrying this variant roughly triples your risk of MS compared to people without it. Other gene variants in the same family also raise risk, though to a lesser degree.
Still, genetics alone aren’t enough. Identical twins share all their DNA, yet when one twin has MS, the other develops it only about 25% to 30% of the time. That gap tells researchers that something in the environment has to interact with genetic vulnerability to actually trigger the disease.
Epstein-Barr Virus Is the Strongest Known Trigger
Nearly all people with MS have been infected with Epstein-Barr virus (EBV), the virus that causes mono. That alone isn’t unusual since most adults carry EBV. But a large population-based study found that people who developed EBV-positive mono had more than three times the risk of later being diagnosed with MS compared to those who didn’t. A separate landmark study tracking over 10 million U.S. military personnel found that MS risk increased 32-fold after EBV infection, making it the single most powerful environmental risk factor identified so far.
The leading theory is that EBV trains certain immune cells to recognize viral proteins that look structurally similar to proteins in the nerve coating (myelin). Once the virus primes the immune system this way, those cells can mistakenly attack the brain and spinal cord. This process, called molecular mimicry, may take years to produce noticeable symptoms.
Where You Grow Up Matters
MS prevalence increases dramatically the farther you live from the equator. Between the equator and 60 degrees north or south latitude, prevalence can rise by as much as 10-fold. In New Zealand alone, rates triple between the northern and southern ends of the country. Canada, Scandinavia, Scotland, and Tasmania consistently report some of the highest rates in the world, while countries near the equator report far fewer cases.
This latitude gradient appears to be established early in life. People who migrate from a high-risk region to a low-risk one before adolescence tend to adopt the lower risk of their new home, while those who move later in life carry the higher risk with them. The most likely explanation ties back to sunlight exposure and vitamin D. People living farther from the equator get less ultraviolet light, which the body needs to produce vitamin D. Blood levels below 20 nanograms per milliliter are considered deficient, and studies of military personnel showed that individuals with the highest vitamin D levels had the lowest MS risk, with risk climbing steadily as levels dropped.
Smoking Raises Risk, Quitting Lowers It
People who have ever smoked face a 41% higher risk of developing MS compared to those who never smoked. The effect is especially pronounced in men: smoking accounts for roughly 19% of male MS cases versus about 11% of female cases. The encouraging finding is that quitting makes a real difference. Among former smokers, the fraction of MS cases attributable to smoking drops to less than 1%, suggesting that the risk largely reverses after stopping.
Researchers believe smoking increases MS risk by irritating lung tissue in ways that activate immune cells and promote inflammation throughout the body. Childhood exposure to secondhand smoke also appears to raise risk, which means the damage can begin well before someone picks up a cigarette themselves.
Obesity During Adolescence
Being obese during childhood or the teenage years roughly doubles the risk of developing MS later in life. Fat tissue is not just stored energy; it actively produces inflammatory signals that can prime the immune system toward the kind of overreaction seen in MS. Obesity also lowers circulating vitamin D levels because the vitamin gets sequestered in fat cells rather than remaining available in the bloodstream, compounding the risk.
Gut Bacteria May Play a Role
The community of microbes living in your intestines appears to influence MS risk in ways scientists are still mapping out. A study published in the Proceedings of the National Academy of Sciences compared gut bacteria from twins where one had MS and the other didn’t. When researchers transplanted intestinal bacteria from the MS-affected twins into specially bred mice, those mice developed an MS-like disease at substantially higher rates than mice receiving bacteria from the healthy twins.
Two specific bacterial species from a family called Lachnospiraceae were implicated. These bacteria expanded dramatically in the gut just before the mice developed disease symptoms, suggesting they may help create the inflammatory conditions that push the immune system toward attacking nerve tissue. This research is still in early stages, but it adds another layer to the picture: the balance of bacteria in your gut, shaped by diet and environment, may contribute to whether a genetically susceptible person actually develops MS.
The Disease Often Brews Silently for Years
MS doesn’t appear out of nowhere. Research into the “prodromal phase” shows that people who go on to develop MS visit their doctors more frequently for up to 10 years before their first recognized MS symptom. In the five years before diagnosis, mental health visits increase by about 50% compared to people without MS. Common complaints during this period include unexplained fatigue, depression, sleep problems, pain, migraines, and bladder or bowel issues.
These symptoms don’t look like MS on their own, which is why they’re usually attributed to stress, mood disorders, or other conditions. Pain is especially common in older individuals during this pre-diagnosis window. The existence of this prodromal phase suggests the immune attack on nerve tissue begins long before it produces the classic signs, like vision problems, numbness, or difficulty walking, that lead to an MRI and a formal diagnosis.
How These Factors Work Together
No one gets MS from a single cause. The current understanding is a layered model: you inherit genes that make your immune system more prone to mistaking nerve tissue for a threat, then environmental factors pile on. An EBV infection in adolescence primes certain immune cells. Low vitamin D from limited sun exposure weakens the body’s ability to regulate those immune cells. Smoking or obesity adds chronic inflammation. Changes in gut bacteria may further tilt the immune balance. At some point, the accumulated weight of these factors crosses a threshold, and the immune system begins attacking myelin.
This explains why MS clusters in certain populations and geographies without being purely genetic or purely environmental. It also explains why the disease is becoming more common in women. The female-to-male ratio has widened over recent decades, now sitting at roughly 3 to 1 globally and reaching 3 to 1 in South-East Asia and the Western Pacific. Since genetics haven’t changed that fast, researchers suspect shifting environmental exposures, including changes in obesity rates, sun exposure habits, and smoking patterns, are disproportionately affecting women’s risk.