Hilchos Taaniyos 2:5

A plague (in halacha 2:1) is when three people die on three consecutive days in a city that has 500 men. If they die on one day or over four days, it is not a plague. If a city has 1,000 men and six people die on three consecutive days, it is a plague. If they die on one day or over four days, it is not a plague. This ratio (three deaths over three days per every 500 men) is followed for cities of any size. The 500 men we use as a baseline does not include women, children, and older men who no longer work.

Hilchos Taaniyos 2:6

If there’s a plague in Israel, residents of other lands should fast for them. If there’s a plague in one country and caravans regularly travel between that country and another, then residents of both countries should fast even if they are far away from one another.