Neuroepidemiology.

Epidemiology is the study of the natural history of disease, which includes its frequency, severity, and course and the identification of "risk factors" that influence these aspects. Neuroepidemiology is that branch of epidemiology dealing with disorders that affect the nervous system. Frequency of disease is best measured by population-based rates, which are ratios of the number of cases to the population at risk, expressed as cases per unit of population. Incidence and mortality rates refer, respectively, to new cases of, and deaths caused by, a disease per unit of time and population. Prevalence rate is the cross-sectional count of cases of a disease present at a given time per unit of population. Comparison of rates often requires consideration of age-specific or age-adjusted rates rather than crude rates, all ages. Course of illness (survival, complications, recurrence) can be measured by life-table methods. The annual incidence for disease and injury of the nervous system is about 2.5% (2,500 per 100,000 population) and the prevalence is about 9.5%. Excluding all traumatic, pain, headache, alcohol, psychiatric, and special-sense disorders, neurological diseases have an annual incidence and prevalence rate, respectively, of 1.1 and 3.6% of the population.