First, some definitions. The incidence of a disease is the number of new cases of a disease that occur over a defined time period. An epidemic, then, is defined as occurring when the incidence of a disease escalates beyond what would normally be expected in a given population. The term pandemic means that a given disease has extended to more than one continent.
There are Common Source Epidemics, usually coming from a contaminated water or food source. This was the case of the cholera outbreak seen in Haiti after the catastrophic magnitude 7.0 earthquake in January of 2010. Or many of you may recall the Pittsfield, Massachusetts “Beaver Fever” Giardiasis outbreak over the three months beginning November, 1985 through January of 1986, when there were 703 confirmed cases of the disease originating from the contaminated City reservoir. (As an aside, Harold Hutchins did a paper of the same title that year—but, true to Monday Evening Club tradition, the subject turned out to be a discussion of the pre-American Revolution trapping industry in the upper Midwest!)