Douglas Goodin, a neurologist in California, has developed a mathematical model based on known research, that clearly shows the genetic background to the development of MS.
He has shown for example, that to develop the disease, one really has to have the genetic susceptibility. However, only a small fraction (less than 2.2%) of the population actually has this genetic susceptibility and can go on to get the disease. Men are more likely to have this susceptibility than women, but if you have the susceptibility, women are more likely to get the disease, around twice as likely. This is due to women being more influenced by environmental factors than men; this also explains why the sex ratio is changing from around 1.6 women to one man to around three women to one man.
The paper goes on to examine the environmental factors known to increase the risk of developing MS for those who have the genetic susceptibility; these are vitamin D level while in utero, vitamin D levels in childhood, and infection with the Epstein-Barr Virus (EBV, the glandular fever virus). While the mathematics is complex, the paper makes very interesting reading.