Both epidemic and outbreak are usually defined as the occurrence of more cases in a place (or population) and time than expected. Of the 75 persons interviewed, 46 were ill with gastroenteritis during a 24-hour time period. This is clearly above the "expected" or background rate of gastroenteritis in a community. The terms "outbreak" and "epidemic" are used interchangeably by many epidemiologists, although some consider the term “outbreak” to refer to a more localized situation, and “epidemic” to refer to a more widespread (and perhaps prolonged) situation. Traditionally, the term “epidemic” has been more frightening to the public than “outbreak,” so most field investigators have used the latter term when talking to the press or public. On the other hand, the term “epidemic” is now at risk of being overused, particularly for social problems to which advocates want to draw public attention and concern. The term "cluster" may be defined as the occurrence of a group of cases in a circumscribed place and time, in amounts that are thought or suspected (by the public or others) to be greater than expected. The cluster is usually based on anecdotal evidence, and often the first task of the epidemiologist is to determine whether the number of cases truly is or is not be greater than expected. As noted above, acute illness in 46 out of 75 persons is clearly above expected rates. If the excess were not as obvious, one should compare the rate with some baseline data. For example, we could compare the observed attack rate with National Health Interview Survey data of 2 episodes of diarrheal illness/person/year. For reportable diseases, most health departments compare the number of current cases with the number in a preceding time period or the comparable time period in the previous year or years.