Multiple sclerosis (MS) occurs because of damage to the nerve fibres of the central nervous system. Your central nervous system consists of your brain and spinal cord and is responsible for controlling every action, conscious and unconscious, of your body.

When you perform an action, your brain sends messages to the appropriate part of your body through the nerve fibres in your spinal cord. These nerve fibres are covered by a substance called myelin. Myelin insulates the nerve fibres and helps carry messages to and from your brain quickly and smoothly. In MS, the myelin around your nerve fibres becomes damaged. This disturbs the messages coming to and from your brain.

MS is an autoimmune condition. This means that 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 process is called demyelination.

Demyelination disrupts the messages travelling along nerve fibres. It can slow them down, jumble them, accidentally send them down a different nerve fibre or stop them from getting through at all.

When the inflammation goes away, it can leave behind scarring of the myelin sheath and sometimes damage to the underlying nerve cell. The progressive types of MS are due to the accumulated damage to these nerve cells.

Want to know more?

• Multiple Sclerosis Trust: What does MS mean for me? (PDF).
• Multiple Sclerosis Society: What is MS?

Why do people develop multiple sclerosis?

MS is an autoimmune condition in which your immune system attacks myelin on the nerve fibres of your central nervous system. It is not understood what causes this autoimmune response, although there are several theories.

Most experts agree that MS is probably caused by a combination of genetic and environmental factors. This means that it's partly due to the genes you inherit from your parents and partly due to 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 that people who are related to someone with MS are more likely to develop it. For example, if your mother has MS, you're 40 times more likely to develop it than normal. However, the chances of MS occurring more than once in a family are still very small, and there is only a 2% chance of a child developing MS when a parent has it.

It's likely 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.

Environmental factors

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

Bacteria and viruses

The reason for the distribution of MS around the world is not fully understood, but it's thought that MS could be triggered by a particular bacteria or virus that thrives in a cooler environment. Some experts believe that a common childhood infection in these cooler countries may disturb the immune system or trigger an autoimmune response in some people, which develops into MS.

As yet, no bacteria or virus has been identified to back up this theory. However, research has shown that people over the age of 15 who move away from countries nearer the equator to a cooler climate have a lower risk of developing MS than those who were born there.

Vitamin D

Other research has focused on the idea that people living further from the equator are exposed to less sunlight and, therefore, have less vitamin D in their body. 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.

Want to know more?

• Multiple Sclerosis Trust: Causes of MS.
• News: Vitamin D linked to MS. 
• NHS Evidence: Vitamin D to prevent MS?