It is estimated that there are currently around 100,000 people with MS in the UK.

MS is most commonly diagnosed in people aged 20 to 40, although it can happen at any age.

For reasons that are unclear, MS is three times more common in women than men.

MS can be a challenging and frustrating condition to live with but new treatments over the past 20 years have considerably improved the quality of life of people with the condition.

MS is not fatal, but some complications that can arise from severe MS, such as pneumonia, can be.

As a result, the average life expectancy for people with MS is around five to 10 years lower than the population at large. This gap appears to be getting smaller, perhaps because of improved medical care.

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

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 gene pool and partly caused by outside factors that may trigger the condition.

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.

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, where the winter season is longer than in the Southern hemisphere.

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 is another factor that appears to increase someone’s risk of developing progressive 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.

Another theory is that common viruses may be involved in the development of MS. The Epstein-Barr virus (EBV) and herpes, which is a very common virus that causes glandular fever, has been the subject of most of the current studies.

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

In around one in five cases of MS, the first noticeable symptom is problems with one of your eyes. You may experience:

• some loss of vision in the affected eye – this can range from mild to severe (total loss of vision occurs in one in 35 cases)
• colour blindness 
• eye pain; usually made worse when moving the eye
• flashes of light when moving the eye

These symptoms are the result of optic neuritis, which is inflammation (swelling) of the optic nerve that transmits visual information to the brain. This normally only affects one eye.

Other visual problems that can occur in MS include:

• double vision 
• eye pain in both eyes
• involuntary eye movements (usually from side to side), known as nystagmus

Bladder problems are common in MS. They may include:

• difficulty emptying the bladder completely
• having to urinate more frequently
• having a sudden, urgent need to urinate which can lead to unintentionally passing urine (urge incontinence)
• having to get up frequently during the night to pass urine (nocturia)

Many people with MS have problems with their bowel.

Constipation is the most common problem, affecting around half of people with MS. They may pass stools much less frequently than normal, and find this difficult.

Bowel incontinence is less common but is often linked to constipation. If a stool becomes stuck, it can irritate the wall of the bowel, causing it to produce more fluid and mucus that can leak out of the anus (back passage).

Around a third of people with MS experience difficulty chewing or swallowing (dysphagia) at some point. In some cases, speech may also become slurred, or difficult to understand.

However, for most people with MS, speech and swallowing symptoms are mild and only last for a few minutes at a time. They are often at their worst during a relapse.

Abnormal sensations can also be a common initial symptom of MS. This can take the form of numbness or tingling in different parts of your body.

Muscles in your arms and legs may also feel unusually weak.

If messages between your brain and muscles are disrupted, this can cause problems with muscle movements. It can cause muscles to contract tightly and painfully (spasm) or your muscles may also become stiff and resistant to movement, known as spasticity.

Around half of people with MS experience pain, which can take two forms:

• Neuropathic pain – caused by damage to the nerve fibres in the brain and spinal cord. It can be a stabbing pain, extreme skin sensitivity, or a burning sensation.
• Musculoskeletal pain – this is not caused directly by MS, but can occur if there is excess pressure on muscles or joints as a result of spasms and spasticity.

MS can affect balance and co-ordination. It can make walking and moving around difficult, particularly if you also have muscle weakness and spasticity. You may experience:

• ataxia – difficulty with co-ordination
• tremor – shaking of the limbs, which is rare, but can be severe
• dizziness and vertigo can happen late on and can make you feel as if your surroundings are spinning

Feeling extremely tired (fatigue) is a common symptom of MS that many people describe as one of the most troublesome.

It is estimated that as many as nine out of 10 people with MS will experience episodes of fatigue.

People with MS have reported feeling an overwhelming sense of weariness where even the most simple physical or mental activity seems to be a tremendous struggle to carry out.
Fatigue may be worse in hot weather, after exercising, or during illness.

Around half of people with MS have problems with thinking, learning and planning (known as cognitive dysfunction). They may experience:

• problems understanding and using language
• a shortened attention span
• problems learning and remembering new things (long-term memory is usually unaffected)
• problems understanding and processing visual information, such as reading a map
• difficulty with planning and problem solving – people often report that they know what they want to do, but can’t grasp the method of how to do it
• problems with reasoning, such as mathematical laws or solving puzzles

Around half of all people with MS experience at least one episode of depression at some point in their life.

It is unclear whether the depression arises from the damage to the brain caused by MS, or due to the stress of having to live with a long-term condition, or both.

Anxiety can also be a problem for people with MS, possibly due to the unpredictable nature of the condition.

Some people with MS can sometimes experience rapid and severe mood swings, suddenly bursting into tears, laughing or shouting angrily for no apparent reason.

MS can have an effect on sexual function.

Men with MS often find it hard to obtain or maintain an erection (erectile dysfunction). They may also find it takes a lot longer to ejaculate when having sex or masturbating, and may even lose the ability to ejaculate altogether.

For women, problems include difficulty reaching orgasm as well as decreased vaginal lubrication and sensation.

Both men and women with MS may find they are less interested in sex than they were before. This could be directly related to the MS, or it could be the result of living with the condition.