If you or a friend or loved one is one of the 400,000 Americans with multiple sclerosis (MS), you may question what caused this chronic central nervous system disease. The short answer is that nobody knows for sure. But there are some pretty good theories. Most experts agree that an abnormal immune system plays a role in MS. You may be born with genes that make you more susceptible to MS, but something in your environment probably triggers those genes to become active. MS is a disease that usually strikes young adults. This leads researchers to think that whatever triggers MS probably occurs before age 15. Possible triggers include viruses and lack of vitamin D. Your chance of getting MS increases if you are female, are of northern European descent, or have a family history of MS. Experts strongly suspect that there's more than a single cause for MS. The Role of Your Immune System Most experts agree that MS is what's called an immune-mediated disease. Normally, a healthy immune system works as the body’s defense system against attackers like viruses or bacteria. In MS, your immune system mistakenly turns against your brain, spinal cord, and optic nerves. The target is mostly the protective insulation around nerve fibers, called myelin. Researchers now know which cells of the immune system lead the attack and they're zeroing in on some of their targets. This knowledge has led to some effective treatments for MS. But researchers still want to know what causes the immune system to go off track in the first place. They also want to learn how to prevent it. The Role of Your Genes Researchers have identified some of the genes that are abnormal in MS. But just because you inherit these genes doesn't mean you will have MS. That’s why MS is not a hereditary disease. If your mother or father has MS, you still have only about a 2-5% chance of developing it. The genetics of MS are still mostly a mystery. Although research has identified some genes, there appear to be many more genes that contribute to MS. And researchers are eager to identify them. Even more important, they would like to know what turns the genes on to cause MS. The Role of Your Environment Something in your environment that happens before age 15 may be the key to what triggers MS. One big MS mystery is why people who live farther from the equator have higher rates of MS. Studies show that if you move closer to the equator before age 15, your chance of developing MS decreases. Some researchers now think vitamin D may be the key to the equator mystery. You need vitamin D for your immune system to work. And your body needs sunlight to make vitamin D. Living father north—where there is less sunlight over the course of a year—may increase your risk of MS. A 2012 study in the journal Neurology looked at vitamin D levels in blood samples from 164,000 people in Sweden since 1975. The study found that people with higher levels of vitamin D were less likely to develop MS. Viral infection at an early age is another possible trigger for MS. Many viruses cause inflammation of the lining around nerve fibers similar to MS. This inflammation is called demyelination. For this reason, researchers suspect several viruses could be MS triggers. This includes herpes, mononucleosis, flu, measles, mumps, hepatitis B, and chicken pox viruses. So far, however, research has not definitely linked any infection to MS. Although there's no cure for MS, it's important to know that there are effective treatments. The more researchers learn about the causes of MS, the closer they may get to curing or preventing the disease. For now, they have lots of good theories to guide their work. Key Takeaways Researchers still do not know the exact cause of MS, an immune-mediated disease that affects the nervous system. Most experts agree that there is a combination of causes. Research has identified some MS genes, but being born with these genes is not enough to cause MS. Triggers that may cause MS in susceptible people include low levels of vitamin D and certain viral infections.