Doctors still don't understand what causes multiple sclerosis . But ongoing research shows that from your genes, to where you live, to the air you breathe , there are many factors in play.

You might be more likely to get MS if you have another autoimmune condition like inflammatory bowel disease , thyroid disease, or type 1 diabetes .

MS is an autoimmune condition. Doctors don’t know why, but something tells your immune system to attack your body. With MS, the focus is on myelin , a fatty substance that covers the nerve fibers in your brain and spinal cord. Its job is to protect them like the plastic that wraps around the wires in your phone charger. When myelin is messed up, your nerves can’t send messages back and forth the way they should. Without their protective coating, your nerves can get damaged, too.

The Environment Raises Your Odds

MS is more likely to affect people who live in certain places and belong to specific ethnic groups. It’s especially common in cooler climates like Scotland, Scandinavia, and throughout northern Europe -- places that are farther from the equator. People who live close to the equator are least likely to get it. In the U.S., it affects white people more than other racial groups.

If you move from a place where MS is rare to a place where it’s common before you’re a teenager, you’ll also be more likely to get it. This suggests that something about the place you live before puberty raises your odds of getting MS. It could be the amount of sunlight in a day. There’s evidence that vitamin D, which your body makes when it’s exposed to sunlight, helps protect you from immune-related diseases.

Are you a smoker? Then you’re also more likely to get MS. And you’ll probably have a worse case that progresses faster than cases for nonsmokers. Quitting can slow the disease down, though, whether you do it before or after you’re diagnosed.

If you smoke and you have clinically isolated syndrome -- a first instance of MS symptoms that lasts for about 24 hours -- you have a greater chance of a second episode and an MS diagnosis.