Multiple sclerosis (MS) is the most frequently seen demyelinating disease, with a prevalence that varies considerably, from high levels in North America and Europe (>100/100,000 inhabitants) to low rates in Eastern Asia and sub-Saharan Africa (2/100,000 population). Knowledge of the geographical distribution of the disease and its survival data, and a better understanding of the natural history of the disease, have improved our understanding of the respective roles of endogenous and exogenous causes of MS. Concerning mortality, in a large French cohort of 27,603 patients, there was no difference between MS patients and controls in the first 20 years of the disease, although life expectancy was reduced by 6-7 years in MS patients. In 2004, the prevalence of MS in France was 94.7/100,000 population, according to data from the French National Health Insurance Agency for Salaried Workers (Caisse nationale d'assurance maladie des travailleurs Salariés [CNAM-TS]), which insures 87% of the French population. This prevalence was higher in the North and East of France. In several countries, including France, the gender ratio for MS incidence (women/men) went from 2/1 to 3/1 from the 1950s to the 2000s, but only for the relapsing-remitting form. As for risk factors of MS, the most pertinent environmental factors are infection with Epstein-Barr virus (EBV), especially if it arises after childhood and is symptomatic. The role of smoking in MS risk has been confirmed, but is modest. In contrast, vaccines, stress, traumatic events and allergies have not been identified as risk factors, while the involvement of vitamin D has yet to be confirmed. From a genetic point of view, the association between HLA-DRB1*15:01 and a high risk of MS has been known for decades. More recently, immunogenetic markers have been identified (IL2RA, IL7RA) and, in particular thanks to studies of genome-wide associations, more than 100 genetic variants have been reported. Most of these are involved in the immune response and often associated with other autoimmune diseases. Studies of the natural history of MS suggest it is a two-phase disease: in the first phase, inflammation is focal with flares; and in the second phase, disability progresses independently of focal inflammation. This has clear implications for therapy. Age may also be a key factor in the phenotype of the disease. In conclusion, France is a high-risk country for MS, but it only slightly reduces life expectancy. MS is a multifactorial disease and the implications of immunogenetics are major. Preventative approaches might be derived from knowledge of the risk factors and natural history of the disease (smoking, vitamin D).