Sporadic: Occurring upon occasion or in a scattered, isolated or seemingly random way.

A disorder that is sporadic is by definition neither an endemic, an epidemic, nor a pandemic: An endemic is present in a community at all times but in low frequency. An endemic is continuous, as in the case of malaria in some areas of the world or as with illicit drugs in certain neighborhoods. An epidemic involves more than the expected number of cases of disease occurring in a community or region during a given period of time. An epidemic is typically a sudden severe outbreak within a region or a group as, for example, AIDS in intravenous drug users. A pandemic is an epidemic that becomes very widespread and affects a whole region, a continent, or the world. For example, AIDS is now pandemic in Africa.

"Sporadic" is from the Greek ""sporadikos" meaning "scattered."