Dean Wingerchuk, MD – Mayo Clinic:
NMO is a rare disease and the rate at which NMO occurs and how many people have NMO are subjects of a lot of research investigations. Doctors who study NMO express the frequency of the disease in two ways called incidents and prevalence. The incidence is the number of people who develop NMO during a period of time, for example, one year and current research suggests that the incidence of NMO is about one to four new cases, new patients every year.
A more meaningful way for many people to understand it is how many people in the population already have the disease, and that estimate is based on per 100,000 population. So if you think of a town that has 100,000 individuals, the prevalence would be the number of individuals in that town with NMO, and currently those estimates range from about four to five cases per 100,000. So that translates in the United States with a population of over 300 million people to probably somewhere between about 12,000 and 15,000 people with NMO.
