The exact reason why someone develops multiple sclerosis (MS) is not known. What is known so far suggests it is caused by a combination of environmental and genetic factors.

Autoimmune condition

MS is an autoimmune condition. This means your immune system mistakes the myelin for a foreign substance and attacks it. The myelin becomes inflamed in small patches (called plaques or lesions), which can be seen on an MRI scan.

This can disrupt the messages travelling along nerve fibres. It can slow them down, jumble them, send them down a different nerve fibre, or stop them from getting through completely.

When the inflammation goes away, it can leave behind scarring of the myelin sheath (known as sclerosis). These attacks can eventually start to destroy the myelin sheath (demyelination), which can damage the underlying nerve fibre.

Why do people develop multiple sclerosis?

It is not understood what causes the immune system to attack myelin, although there are several theories. Most experts agree that MS is probably caused by a combination of genetic and environmental factors. This means it's partly caused by genes you inherit from your parents and partly caused by outside factors that may trigger the condition.

Genetic factors

MS is not defined as a genetic condition because there is no single gene that causes it. It's not directly inherited, although research has shown people who are related to someone with MS are more likely to develop it.

Researchers have found that if one twin develops MS then the second twin has around a one in four chance of also developing MS.

The chance of a brother, sister or child of a person with MS also developing MS themselves is less than one in 30.

It's possible that different combinations of genes make developing MS more likely, and research into this is continuing. However, genetic theories cannot explain the wide variation in occurrences of MS throughout the world.

Sunlight and vitamin D

Research into MS around the world has shown that it's more likely to occur in countries far from the equator. For example, MS is relatively common in the UK, North America and Scandinavia, but rare in Malaysia or Ecuador.

It’s possible that people living further from the equator are exposed to less sunlight and, therefore, have less vitamin D in their bodies. Some studies have found a link between lower levels of vitamin D and incidence of MS.

Some researchers have suggested that vitamin D supplements may reduce the risk of MS. However, this has not been proven.

Smoking

Smoking is another factor that appears to increase someone’s risk of developing MS. It is not yet clear exactly why this is, although one theory is that the chemicals in the cigarette smoke affect the immune system.

Viral infection

Another theory is that a virus may be involved in the development of MS. The Epstein-Barr virus (EBV) has been the subject of most of the current studies.

MS is thought to be an autoimmune condition, where the body's own immune system attacks healthy tissue. One possible explanation is that a virus might cause a strong response from the immune system, which leads it to target healthy parts as well as the virus.

More research is needed to further understand how EBV may increase the risk of developing MS.