Multiple sclerosis affects roughly 2.9 million people worldwide, and it doesn’t strike randomly. Certain groups face significantly higher risk based on their age, sex, genetics, geography, and even past infections. While anyone can develop MS, the typical person diagnosed is a woman in her early 30s living in a temperate climate far from the equator.
Age at Onset
MS most commonly appears between ages 20 and 40, with the average age of onset around 33 to 34 years old. Recent population data shows a shift toward a two-peak pattern: one cluster of diagnoses around age 30 and a second around ages 40 to 45. This means a growing number of people are being diagnosed in middle age, not just their twenties and thirties.
That said, the range is wide. Cases have been documented as early as age 4 and as late as 76. Children under 18 account for about 3 to 5% of all MS cases worldwide, a category known as pediatric-onset MS. Late-onset cases (after 50) are less common but do happen, and they can be harder to diagnose because symptoms overlap with other age-related neurological conditions.
Women Are Affected Far More Than Men
For every man diagnosed with MS, roughly two to three women receive the same diagnosis. Across large international studies, the overall female-to-male ratio sits around 2.4 to 1. That gap has been widening over time. In earlier decades, the ratio was closer to 2.35 women per man; in more recent birth cohorts, it has climbed to about 2.73 to 1.
The reasons aren’t fully understood, but hormonal differences and how the female immune system responds to infections and environmental triggers are likely contributors. The widening gap over time suggests that whatever environmental factors are driving the increase in MS cases may be hitting women harder than men.
Race and Ethnicity
MS was historically considered a disease of white populations, and prevalence data still reflects a higher burden in that group. In the United States, the estimated prevalence per 100,000 adults is 374.8 for white individuals, 298.4 for Black individuals, 197.7 for people of other non-Hispanic racial and ethnic backgrounds, and 161.2 for Hispanic individuals. White individuals make up about 77% of U.S. MS cases, with Black individuals representing roughly 10%.
These numbers tell only part of the story. Black individuals with MS tend to experience more aggressive disease progression and greater disability compared to white individuals with MS. So while overall rates are lower, the impact per person can be more severe. Awareness in non-white populations has also historically been lower, which may contribute to delayed diagnosis.
Geography and Latitude
Where you live, or where you grew up, matters. The traditional pattern shows MS prevalence climbing the farther you get from the equator. Near the equator, prevalence runs about 5 to 10 per 100,000 people. Above 59 degrees north latitude (think Scandinavia, northern Canada), it jumps to around 200 per 100,000. A similar gradient appears in the Southern Hemisphere.
This pattern has become less clear-cut in recent years, particularly for new diagnoses in Europe and North America, where MS rates have been rising broadly. The latitude gradient for overall prevalence, though, still holds. Countries in northern Europe, Canada, and parts of Australia and New Zealand consistently report among the highest rates globally. The leading explanation is sunlight exposure and vitamin D: people living at higher latitudes get less ultraviolet radiation, produce less vitamin D, and appear to face greater MS risk as a result.
Vitamin D and Sun Exposure
Low vitamin D levels are one of the most studied environmental risk factors for MS. In studies of people with early MS symptoms, over half had vitamin D deficiency. Those with the lowest levels (bottom 10th percentile) faced roughly three times the risk of progressing to a full MS diagnosis compared to those with adequate vitamin D, after adjusting for other risk factors.
This connection helps explain the latitude gradient. It also raises practical questions about supplementation, though vitamin D alone has not been proven to prevent MS. What the data does show is that growing up in a sun-poor environment during childhood and adolescence appears to set the stage for higher risk later in life.
The Epstein-Barr Virus Connection
Nearly all people with MS test positive for past Epstein-Barr virus (EBV) infection, the virus that causes mononucleosis. By contrast, the risk of MS in people who have never been infected with EBV is extremely low. A landmark study of U.S. military personnel found that young adults who contracted EBV saw their MS risk increase 32-fold compared to those who remained uninfected.
Most adults (over 90%) have been infected with EBV at some point, so the virus alone doesn’t cause MS. But it appears to be a necessary precondition. Something about how the immune system responds to EBV, likely in combination with genetic vulnerability and other environmental factors, sets the stage for the immune system to begin attacking the nervous system.
Genetic Risk Factors
MS is not directly inherited, but genetics play a significant role in who develops it. The strongest known genetic link involves a specific variation in the immune system’s cell-recognition genes. People who carry this variant (known as HLA-DRB1*1501) face roughly 2.5 to 3 times the risk of developing MS compared to those without it. This variant is most common in people of Northern European descent, which partially explains the higher rates in that population.
Beyond that single gene variant, researchers have identified over 200 smaller genetic risk factors that each contribute a modest amount. Having a first-degree relative with MS (a parent, sibling, or child) raises your risk to roughly 2 to 4%, compared to a general population risk of about 0.1 to 0.3%. Identical twins of someone with MS have about a 25 to 30% chance of developing it themselves, reinforcing that genes matter but don’t tell the whole story.
Smoking and Obesity
Smoking increases both the risk of developing MS and the speed at which it progresses. Current smokers with MS face about 20 to 27% higher odds of worsening disability, physical decline, and cognitive decline compared to nonsmokers. The effect is consistent across multiple measures of disease progression.
Obesity, particularly during adolescence, is also linked to higher MS risk. Among people already diagnosed, having a BMI above 30 is associated with a 37 to 45% greater likelihood of reaching significant disability milestones compared to those at a lower weight. Smoking and obesity together compound the problem, making disease management harder on multiple fronts. Both are modifiable, which makes them important targets for anyone concerned about their risk.
Putting the Risk Factors Together
No single factor causes MS. The current understanding is that MS develops when a genetically susceptible person encounters the right combination of environmental triggers. A typical high-risk profile might look like this: a woman of Northern European descent who grew up far from the equator, had mononucleosis as a teenager, has low vitamin D levels, smokes, and carries the HLA-DRB1*1501 gene variant. But plenty of people who fit that description never develop MS, and some people with almost none of those risk factors do.
What makes MS unpredictable is the interaction between these factors. EBV infection in someone with the right genetic makeup and low vitamin D may tip the immune system toward attacking nerve insulation, while the same infection in someone without those vulnerabilities causes nothing more than a sore throat and fatigue. The disease ultimately results from a cascade of unlucky overlaps, not any single cause.