I wrote this else where, but I think he's joking, but the explanation is:

"People used to believe that Jews kidnap babies, eat children, cast spells to affect the weather, poison the water, cause plagues etc.... The last one is based on reality, believe it or not, because remember that mediaeval Europe was known for it's lack of hygiene, which proliferated disease, but Jews have a code of hygiene, the Koscher law, that they literally follow religiously. Literally. Religiously. In addition they were often separated from other populations, which further decreases the change of infections, the end result is that to the peasant's perspective Jews were mysteriously never affected by plagues... which is suspicious to a primitive mind."