Modern day America is quite different from other countries in prior times. There is a level of tolerance that is unprecedented anywhere else in history.
Two hundred years ago, there was a stigma to a people that lived within a country but had a separate religion, ate only special foods (so did not interact socially with their neighbors), wore different clothes, etc. The Jews, desiring to follow their religion, could only find suitable food in their own community and thus tended to live together by choice – although, as has been said, in many cases the ghetto was forced upon them. That became circular, they were “different” because they lived apart, and they were forced to live apart because they were “different.”
They were often not allowed to practice certain professions (How could a good Christian be treated by a Jewish doctor? healing was, after all, mostly a matter of God’s blessing and superstition in the days before antibiotics.) This pushed them into “undesired” professions, such as money-lending. As another example, in the early 1900s America, many Jews went into the entertainment business – vaudeville and movie-making – which was looked down on by society. This also became circular: the Jews were not allowed in many professions, so became money-lenders, hence were accused of “controlling” the banks. There was also a practical side to that sort of hatred – if a Christian owed money to a Jew, and that Jew accidentally got killed in an anti-Jewish riot, then the debt obviously disappeared. Shakespeare’s audiences thought it wonderfully amusing that Shylock the money-lender is cheated out of being repaid for his loans.
This circularity can be seen in most of the areas that MHand listed above.
The hatred of Jews has had a much longer period of time to fester than most other ethnic or religious hatreds. While one can trace it (as Izzy does, back to Biblical times), I think modern anti-semitism probably starts with selected New Testament passages. The early Christians sought to work within Judaism but were rejected, and so turned to the pagan world for converts, and lashed out fairly viciously at the stubborness of the Jews for clinging to their religion. Mohammed did the same, when he founded Islam, and for the same reasons.
From then on, of course, the circularity starts in. The Jews are hated and persecuted for being different, and that pushes them together and makes them more clannish, which maeks them perceived as “different” which makes persecution easier.
Through the last 2,000 years, there have been a few havens where Jews lived in peace and harmony and acceptance with the predominant society. Surprisingly, most of these periods were within Islamic countries. In the last 150 years, America has been the ONLY country that allowed Jews to live and worship within a range of acceptance. Granted, there was discrimination against Jews, but there wasn’t out-and-out murderous persecutions. This is why there is a large and thriving Jewish community in the United States. This is also why Jews in the U.S. have usually been in the forefront of civil rights movements.