Photo: Woman with MS waiting for a bus with her husband

Causes of MS

No one knows for sure why people get MS. It's likely to be due to a mix of genes, something in your environment and some lifestyle factors.

Genes and family history

MS is not directly inherited from parent to child. There's no single gene that causes it. Over 100 genes might affect your chances of getting MS. But genes are only part of the story.

MS can happen more than once in a family, but it's much more likely this will not happen. There's only about a 1.5% chance of a child developing MS when their mother or father has it (that means around one in 67 get it). There's only around a 2.7% chance that you'll get MS if your brother or sister has it (around one in 37 get it). In 2014 a very large study found that MS may be even less likely to be passed on than these figures suggest.

> Read more about genetics and MS

Environmental factors

MS is less common in tropical countries near the equator that get lots of sunshine. More people have MS in places further away from the equator like Britain, and that's true no matter what your ethnic background is. Other countries where MS is more common include Canada, the US, Scandinavia, southern Australia and New Zealand.

Infections

There's evidence that some viruses, and maybe bacteria, can help trigger MS.

A common virus called Epstein Barr virus (it causes glandular fever) has been linked to MS. Most people have had this virus but they never get MS. This shows that, like genes, infections might play a role but they aren't the whole story.

Vitamin D

There's more and more evidence that low levels of vitamin D, especially before you become an adult, could be a factor in why people get MS.

Our skin makes most of our vitamin D when we're out in the summer sun. We also get some from food like oily fish, eggs, spreads and breakfast cereals with added vitamin D in them. You can also get extra vitamin D from supplements (but too much can be harmful).

A blood test can show if your levels of vitamin D are low.

> Read more about vitamin D and MS

Lifestyle

Smoking

Studies show you're more likely to get MS if you smoke. It might be because the chemicals in cigarette smoke affect your immune system. If you have MS in your family, your risk of getting it too goes up if you're exposed to passive smoking (breathing in other people's smoke).

If you have relapsing MS and you stop smoking it can slow down how fast your MS might change to secondary progressive MS.

Obesity

Studies show that getting MS could be linked to being very overweight (obese), especially when you were a child or young adult. This might be because obese people are often low in vitamin D. Obesity can also make your immune system overactive and cause inflammation in your body. There may be other reasons we don't understand yet.

Of course, not all people who are very overweight get MS, and having MS doesn't mean you are or were obese. But if your risk of getting MS is on your mind, perhaps because a close relative has it, then your weight is a risk factor you can change.