Researchers in Tasmania have provided important data over many years on the incidence and prevalence of MS. Their latest study analysed the incidence (how many new cases occur per capita annually) and the prevalence (how many people have MS per head of population at any given time).
Their data confirm findings from other parts of the world indicating a markedly increasing incidence and prevalence of this disease. The number of new cases per year per capita roughly doubled from the 1950s to the 2000s. Prevalence tripled from 1961 to 2009, going from about one in 3,000 people to about one in 1,000 people in Hobart. An editorial by Dr George Ebers from Oxford suggested that this was unlikely to be just due to detecting more cases because of MRI, because most of the change happened before MRI was available. It is important to note that more cases were being diagnosed in the elderly, and the increased longevity of the population may have partly explained the increase in incidence and prevalence, as more people lived long enough to get the disease, and the age of the cohort increased. The death rate from MS over the period however halved.
It is clear that environmental influences confer the majority of the risk of developing MS. It is likely that environmental factors over the last half of the last century were changing in a way that increased the risk of getting MS. It seems likely these factors are the common environmental triggers for MS discussed on this website, such as sun avoidance, high saturated fat diet, omega 3 deficiency and so on.