In the wake of the horrific knife attack on Saturday during a Hanukkah celebration in Monsey, N.Y., and following a recent spate of other anti-Semitic assaults in New York City and elsewhere in the United States, we must ask and answer two key questions: “Why now?” and “What can be done to stop such incidents?”

Let’s start with “Why now?” Why, when American Jews have felt unmatched levels of inclusion and equality, and when, unlike in previous generations, Jews can be found in every sphere of American society, is anti-Semitism making a comeback?

It is important to remember that anti-Semitism has been called the world’s oldest social disease. It dates back millenniums. It has taken many forms — religious and racial, political and social. Its durability and ability to reinvent itself should never be underestimated. Even here in the United States, it never entirely vanished.

The resurgence of anti-Semitism could be a result, in part, of the vanishing legacy of the Holocaust. Recent surveys reveal abysmal levels of knowledge among young people about what happened to the Jewish people in the Second World War. There is far too little understanding about the slippery slope from the Nazi dehumanization of the Jews in 1933 to the Final Solution nine years later.