Mishnayos Taanis Perek 3 Mishnah 4

The mishna continues: And likewise, if a city is afflicted by pestilence or collapsing buildings, that city fasts and cries out, and all of its surrounding areas fast but they do not cry out. Rabbi Akiva says: They cry out but they do not fast. The mishna inquires: What is considered a plague of pestilence? When is a series of deaths treated as a plague? The mishna answers: If a city that sends out five hundred infantrymen, i.e., it has a population of five hundred able-bodied men, and three dead are taken out of it on three consecutive days, this is a plague of pestilence, which requires fasting and crying out. If the death rate is lower than that, this is not pestilence.

וְכֵן עִיר שֶׁיֶּשׁ בָּהּ דֶּבֶר אוֹ מַפֹּלֶת, אוֹתָהּ הָעִיר מִתְעַנָּה וּמַתְרַעַת, וְכָל סְבִיבוֹתֶיהָ מִתְעַנּוֹת וְלֹא מַתְרִיעוֹת. רַבִּי עֲקִיבָא אוֹמֵר, מַתְרִיעוֹת וְלֹא מִתְעַנּוֹת. אֵיזֶהוּ דֶבֶר, עִיר הַמּוֹצִיאָה חֲמֵשׁ מֵאוֹת רַגְלִי, וְיָצְאוּ מִמֶּנָּה שְׁלשָׁה מֵתִים בִּשְׁלשָׁה יָמִים זֶה אַחַר זֶה, הֲרֵי זֶה דֶבֶר. פָּחוֹת מִכָּאן, אֵין זֶה דֶבֶר: