Epidemiology ‐ 2

MS has been described in various populations and geographical regions with different frequencies. Generally, the most commonly used frequencies in epidemiological studies of diseases are prevalence and incidence. As described in the next subsection (etiopathogenesis), regional differences in MS prevalence and incidence have contributed to formulating a hypothesis on its pathogenesis. Prevalence is traditionally defined as ameasurement of the proportion of “events” in a population at a given point of time. “Event” is defined as the occurrence of any phenomenon that can be discretely characterized, for example: infection, presence of antibodies, pregnancy. In epidemiology, diseases or infections are the most commonly used events, and therefore prevalence can be defined as the proportion of people in a population (of a state, region, province) that, at any given time, are affected by the disease. Incidence, on the other hand, measures the number of new cases of a disease occurring during a given period (for example, in a month or a year) and identifies the risk (i.e. probability) of developing the disease in the considered population. Since incidence indicates a change in a quantity (new people affected) comparedwith the change in another quantity (time), it is considered to be a dynamic measure. Epidemiological studies, as well as other studies, are made difficult by the particular nature of MS. In fact, epidemiological researches aimed at formulating and testing etiopathogenetic hypotheses demand levels of diagnostic accuracy