One thing I thought perhaps I should provide more background on - because BloodAndFire mentioned the Blood Libel - and while many may know what that is a reference to, many may not.
The blood libel was a form of conspiracy theory - and like many of the most potent conspiracy theories, it centred on children (because threats to children are, understandably, what raises the greatest emotions -and so it is most effective at by-passing the rational brain).
Under this particular conspiracy theory, Jews were accused of killing non-Jewish children - often to use their blood in some sort of imaginary Jewish ritual. Very often there really was a dead or missing child - children unfortunately die or go missing from all sorts of causes - including murder (and health and safety was not highly developed in these places)- and when there is no known culprit, people want certainty, and they want revenge, and blaming the Jews and going off and murdering Jews and burning down their houses and looting and pillaging and raping can be very attractive. As we are starting to see from the internet, it is extraordinary what people can be persuaded to believe when things aren't going so well for them - and this is in the modern era, with modern science and widespread literacy etc. (Of course to the extent there were real murderers, it is convenient if nobody investigates you because they are convinced it is the Jews).
England has the rather dubious privilege of being the first recorded blood libel - William of Norwich in 1144. The Jews of Norwich were charged with ritual murder after the body of a young boy was discovered stabbed to death in the woods. In this case, the Jews of Norwich were alleged to have "bought a Christian child [the 'boy-martyr' William] before Easter and tortured him with all the tortures wherewith our Lord was tortured, and on Long Friday hanged him on a rood in hatred of our Lord."
I confess though that while a lot of people claim this as the first blood libel - I strongly suspect this was not a first - just that England had better documentation and court systems - so we have better records of it.
Like witch hunting - where alongside the documented cases there were almost certainly very many other cases, particularly in more lawless lands, where viligante action bypassed the legal processes and hence were never formally recorded. People want someone to blame, and to be angry at, and to then act, so that they can feel safe, and they are capable of being completely irrational in so doing.
Another conspiracy theory linked to this is that of well poisoning - this was particularly prevalent in areas where the black death was - because not understanding the nature of plague and the way it spreads, conspiracy theorists look for someone to blame. And of course, the black death killed many, many people, so their were lots of people very upset and angry and irrational and desperate to do what they could to eliminate it. So killing Jews because they had poisoned the wells, causing the black death, was also widespread. (BTW Jews may have had at least a little better protection against certain plagues, because Jewish ritual does involve a lot of hand washing - including before a meal of bread - not that Jews had any more idea that this was protective against disease than anybody else).
The Jewish sense was very much of a constant threat well beyond the formally documented cases such as the blood libel of William of Norwich. Nor was this limited to Christian Europe. There are documented cases in Damascus (Syria) in 1840 and Shiraz (Iran) in 1910 - but again, this is about modern documentation, not about the Jewish sense of how often these occurred. What is notable about the two Middle Eastern cases is that, with colonisation, there was clearly a cross fertilisation of conspiracy theories.
Many of you may well know most if not all of this, but I find that not everybody does so just worth a recap.