Yiddish has its routes in tenth-century Germany, where Jews from France and Northern Italy established communities, developing a language with elements of German, Laaz, Hebrew, and Aramaic. The language spread and changed as Jews migrated eastward to escape persecution. Before World War II there were 11 to 13 million Yiddish speakers (Jacobs, 2005), but The Holocaust led to a dramatic reduction in the number of speakers.
There are now roughly 500,000 people speaking Yiddish as their first language, in at least 15 territories. Israel has the largest population, followed by the United States. Most of the remainder are in Eastern-European territories.
Reference: Jacobs, N. G. (2005) Yiddish: a Linguistic Introduction. Cambridge University Press, Cambridge.