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 trigger is responsible. Instead, MS develops from a combination of genetic susceptibility, viral infection, and environmental factors that together push the immune system toward this self-destructive behavior. Understanding how these pieces fit together helps explain why some people develop MS and others don’t.
How the Immune System Damages Nerves
Nerve fibers are wrapped in a fatty insulating layer called myelin, which allows electrical signals to travel quickly between the brain and the rest of the body. In MS, immune cells that normally fight infections cross into the brain and spinal cord and begin destroying this insulation, along with the cells that produce it.
The attack involves multiple types of immune cells working in concert. Certain white blood cells called T-cells become activated against proteins found in myelin. Once they cross into the central nervous system, they trigger a cascade of damage. Some T-cells release toxic compounds that kill the myelin-producing cells directly. Others recruit additional immune cells to the site. B-cells produce antibodies that tag myelin for destruction, activating a system that punches holes in cells and flags debris for cleanup. Macrophages then move into the center of the developing lesion and consume the damaged myelin, which is why brain scans of people with MS show characteristic patches of scarring.
This process strips the insulation from nerve fibers, slowing or blocking the electrical signals that control movement, sensation, vision, and cognition. When inflammation subsides, the body can partially repair the damage, which is why many people with MS experience symptoms that come and go in relapses. Over time, repeated attacks and incomplete repair lead to permanent nerve damage.
The Genetic Component
MS is not directly inherited, but genes play a significant role in determining who is vulnerable. The strongest genetic link involves a specific immune system gene called HLA-DRB1*15, which nearly triples the odds of developing MS (with an odds ratio of 2.84). This gene helps immune cells distinguish the body’s own proteins from foreign invaders. Variants like this one appear to make the immune system more likely to mistake myelin proteins for threats.
More than 200 other gene variants have been linked to MS risk, most of them involved in immune function. Each one adds a small amount of risk on its own, but their combined effect is substantial. Having a first-degree relative 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 about a 25 to 30 percent chance of both developing the disease, which confirms that genes matter but are far from the whole story.
Epstein-Barr Virus as a Trigger
The strongest environmental risk factor identified so far is infection with the Epstein-Barr virus (EBV), the common virus that causes mononucleosis. A landmark 2022 study tracking over 10 million military personnel found that EBV infection increased the risk of MS 32-fold, while no other virus showed a similar effect. Nearly all people with MS have been infected with EBV, compared to about 95 percent of the general adult population.
The mechanism appears to involve a case of mistaken identity. About 25 percent of people with MS carry antibodies that react to both an EBV protein called EBNA-1 and a protein found on the surface of brain cells called GlialCAM. These two proteins share similar structural features. When the immune system mounts a response against EBV, it may inadvertently train itself to also attack this brain protein. Immune cells carrying this cross-reactive targeting have been found in the spinal fluid of MS patients, providing a direct link between the viral infection and the immune assault on the central nervous system.
Vitamin D and the Latitude Effect
MS becomes more common the farther you live from the equator. For every degree of latitude you move away from the equator, MS prevalence increases by roughly 6 to 7 cases per 100,000 people. Countries like Canada, Scotland, and Scandinavia have some of the highest rates in the world, while MS is relatively rare near the equator.
The leading explanation is vitamin D, which is produced in the skin during sun exposure and plays an important role in regulating immune function. Research from Harvard Medical School found that people with vitamin D blood levels in the top 20 percent (above roughly 40 ng/mL) had a 62 percent lower risk of MS compared to those in the bottom 20 percent (below about 25 ng/mL). People living at higher latitudes get less ultraviolet light for much of the year, leading to lower vitamin D levels on average. Migration studies support this connection: people who move from high-risk to low-risk regions before adolescence take on the lower risk of their new home, suggesting that sun exposure during childhood and teenage years is particularly important.
Smoking and Chemical Exposures
Smoking increases the risk of developing MS by about 41 percent compared to never smoking. It also accelerates progression in people who already have the disease, making it one of the most actionable risk factors. The lungs are a major site of immune activation, and inhaling irritants may prime immune cells in ways that promote autoimmune responses in the brain.
Occupational exposure to organic solvents, including paint, varnish, and industrial degreasers, raises MS risk by about 50 percent. What makes these findings particularly striking is how dramatically the risks compound when combined with genetic susceptibility. People exposed to solvents who also carry high-risk MS genes are nearly seven times more likely to develop the disease than people with neither risk factor. Add smoking to that combination and the risk jumps to 30 times higher. Researchers estimate that the interaction between MS genes and solvent exposure alone accounts for roughly 60 percent of MS risk in exposed individuals, a finding published by the American Academy of Neurology.
Obesity in Childhood and Adolescence
Being significantly overweight during childhood appears to independently increase MS risk, particularly for girls. Extremely obese girls have nearly four times the odds of developing MS or early MS-like symptoms compared to girls at a normal weight. The connection likely involves the chronic low-grade inflammation that accompanies excess body fat, along with lower vitamin D levels commonly seen in people with obesity (vitamin D is fat-soluble and gets sequestered in fat tissue). This is another window where early-life conditions seem to set the stage for disease that may not appear until years or decades later.
The Role of Gut Bacteria
The trillions of bacteria living in your digestive tract play a surprisingly important role in training and regulating your immune system, and researchers have found consistent differences in the gut microbiomes of people with MS. A study of 81 pairs of identical twins where only one twin had MS found 51 bacterial species present at different levels between the affected and unaffected twin. Bacteria that produce short-chain fatty acids, which help calm immune responses, were depleted in the twins with MS. Meanwhile, certain bacteria from a family called Lachnospiraceae were elevated, and when researchers transferred these bacteria into genetically susceptible mice raised in germ-free conditions, the mice developed MS-like disease.
This doesn’t mean gut bacteria cause MS on their own. But in someone who is already genetically predisposed, an imbalanced gut microbiome may tip the immune system further toward the kind of overactivity that targets myelin.
Why Women Are Affected More Often
Globally, about 73 percent of people diagnosed with MS are women, making the female-to-male ratio roughly 3 to 1. This gap has widened over the past several decades, suggesting that environmental or lifestyle changes are interacting differently with female biology. Sex hormones are a likely factor: estrogen and progesterone influence immune function in complex ways, and MS relapses often decrease during pregnancy (when these hormones are high) before increasing again after delivery. The X chromosome also carries a disproportionate number of immune-related genes, and having two copies may contribute to the heightened immune reactivity that makes autoimmune diseases more common in women generally.
How These Factors Work Together
No single cause explains MS. The current understanding is that the disease requires a genetic foundation, usually involving immune system genes like HLA-DRB1*15, combined with one or more environmental triggers. EBV infection appears to be a near-universal prerequisite, but most people infected with EBV never develop MS. The difference likely comes down to additional factors: low vitamin D during key developmental windows, smoking, chemical exposures, obesity in youth, or shifts in gut bacteria that nudge the immune system past a tipping point.
This layered model explains many of the patterns seen in MS. It explains why the disease clusters in families but doesn’t follow simple inheritance. It explains why MS is more common in northern countries, in women, and in people with certain occupational exposures. And it explains why the disease can appear to strike unpredictably, since any given person carries a unique combination of risk factors, many of which were set in motion years before the first symptom appeared.