There's nothing more frustrating than doing a jigsaw puzzle and discovering there's a piece missing. Or worse still, not even knowing how many pieces might be missing. Though it's estimated that 2.3 million people in the world are living with MS, experts are still trying to fit all the pieces of the MS jigsaw together to give us the whole picture - and finally solve the mystery of why some people get multiple sclerosis but most don't. People who study patterns of disease - who officially go by the job title "epidemiologist" - have identified a number of pieces of the puzzle that may influence your chances of getting MS.

What is the average age of MS diagnosis?

MS can occur at any age but is most commonly diagnosed in people who are between 20 and 40 years of age.

What is the connection between hormones and MS?

Women are twice as likely (or more) to develop MS as men are, and this could be in part due to genes. A study published in the journal Neurology in 2011 found that women with MS were more likely to have a specific genetic characteristic linked to the disease than men. Another research study also found that women were more likely to pass the genetic characteristic to their daughters than fathers were to pass it to their sons.

One study also suggests that hormonal changes appear to play a role in how the condition develops, too; for women the disease may strike later in those who have children earlier or who use the oral contraceptive pill. Men, meanwhile, develop MS slightly later than women. Recent research has also linked low levels of testosterone, a male hormone, with a potentially worse disease outcome.

Does where you live increase your chances of developing MS?

The incidence of MS generally increases the further away from the equator you get (with a few exceptions). Parts of Asia, Africa, and America that lie on the equator have extremely low levels of MS, for example, while countries such as Canada and Scotland have particularly high rates. Interestingly though, studies show that if you move from an area with higher risk to one of lower risk before adolescence, you acquire the risk of your new home. This suggests that exposure to some environmental agent(s) before puberty may predispose a person to MS. Some scientists believe this phenomenon is linked to vitamin D levels, as there is less sunshine in northern countries, so the body is unable to make enough vitamin D.

Is there a connection between race and MS?

Certain ethnic groups have a greater tendency to develop MS, such as those from Northern European ancestry. People of Southeast Asian, African, or Inuit descent have the lowest risk. Interestingly, some ethnic groups have a markedly lower prevalence of MS despite living in countries where MS is relatively common, for example, the Inuit in Canada, the Sami of Northern Norway, and the Maoris of New Zealand. This may be explained by genetic or lifestyle and cultural factors, or a combination of the two.

What is the role of genetics in MS?

MS isn't classified as a genetic disease, as no single abnormality of the gene has been identified as being responsible for it - although as we've just seen, there is evidence to suggest that genes are somehow involved. Around 15% to 20% of people living with MS have a relative with the disease. But studies using identical twins show that if one identical twin develops MS, the risk for the other twin is only about 30%.

Some researchers think that MS develops because a person is born with a genetic predisposition, which can trigger the disease when outside factors are also present.

Does your lifestyle increase risk of developing MS?

As MS is an autoimmune disease, it stands to reason that anything that affects the immune system can alter your susceptibility to developing it. For example, obesity and smoking are both associated with an increased risk of developing MS.

In one recent study, researchers found that participants who were obese at the age of 18 were twice as likely to develop MS later in life, compared with individuals of the same age who were not obese. Research suggests this could be because obesity increases levels of the hormone leptin, which promotes inflammation in the body.

Dietary sources of vitamin D, such as fatty fish, may also play an important role. A recent Swedish study found that frequent fatty fish intake may be associated with a decreased occurrence of MS. As with genes, however, there is no one clear trigger of the disease, and research into each of these factors and the impact they have is ongoing.

What does this all mean?

MS is clearly an incredibly complex disease. The jigsaw pieces that we currently know about seem to suggest that some people are genetically predisposed to the disease, which can then be triggered by environmental factors, including low vitamin D levels, changes in hormone levels and even lifestyle factors such as obesity and smoking. The more we understand about these risk factors, the closer scientists will be to completing the puzzle.

If you are concerned about your risk of MS, or that of your children, your doctor will be happy to discuss all your questions.