Maseches Taanis, Chapter 3, Mishnah 4:
"Vechein ir sheyeish bah dever umapoles" - a city in which an epidemic has broken out, or in which a collapse has occurred: an earthquake or buildings that caved in.
"Osah ha'ir mis'annah umasra'as" - that city itself fasts with the stringent form of the fast, and it also sounds the alarm.
And what is the law regarding the cities around it?
"Vechol sevivoseha mis'annos velo masri'os" - the surrounding cities must fast, but they do not blow the shofar or the trumpets.
"Rabbi Akiva omer: masri'os velo mis'annos" - on the contrary, they sound the alarm but do not fast.
"Eizehu dever?" - what is the measure at which an outbreak is considered an epidemic?
"Ir hamotzi'ah chameish me'os ragli" - a city that has five hundred people, adult men, while women, children and the elderly are not included in this count.
"Veyatze'u mimenah sheloshah meisim bishloshah yamim zeh achar zeh - harei zeh dever" - if three people died there on three consecutive days, one after another, this is an epidemic relative to that population.
"Pachos mikan - ein zeh dever" - any measure less than this is not considered an epidemic, but rather the normal course of events.
In summary: in this Mishnah we learned that a city in which an epidemic or a collapse occurred fasts and sounds the alarm, while the cities around it fast and do not sound the alarm; and according to Rabbi Akiva, they sound the alarm and do not fast. We also learned the measure of an epidemic: a city that has five hundred people in which three died on three consecutive days, one after another, this is an epidemic, and less than this is not an epidemic.