This piece originally appeared in The New  York Times.

By Nita Lowey and David Harris

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.

Social media may also be playing a role. In the past, anti-Semites lived in small ideological circles with limited reach. Now the internet amplifies the voices and influence of these otherwise marginal groups.

Another factor could be the declining confidence in liberal democracy and its core value of pluralism. Our nation has made considerable progress in the social inclusion of minority groups. But that progress also poses a threat to those who are bewildered or angered by these social changes and who prefer mutual rancor to mutual respect.

There’s also the “copycat” phenomenon — when someone else’s hateful actions, and the publicity they engender, spur others who seek notoriety and attention. This includes offenses like scrawling swastikas or ugly slogans on synagogue walls; assaulting people on the street in Brooklyn and Manhattan who are “identifiably” Jewish; and murdering them, as with the synagogue shootings in Pittsburgh last year and in Poway, Calif., in April, and the kosher supermarket shooting this month in Jersey City, N.J.

Which brings us to the second of our two questions: How to respond?

First, we need to recognize the problem for what it is: an epidemic. We are no longer talking about isolated, occasional actions — bad enough as those are — but a regular phenomenon. Like an epidemic, it must be treated comprehensively, addressing root causes.

Second, we must acknowledge that there are multiple ideological sources feeding this paroxysm of hate; it is not a result of a single political outlook. Some critics wish to exploit the issue to undermine their political opponents. That is no way to deal with anti-Semitism. There is no one-size-fits-all profile for the perpetrators of these attacks.

Third, we cannot allow this situation to become the “new normal,” as if attacks on Americans because of their religious or ethnic identities are now an expected part of our everyday lives. No, they are not. These attacks violate everything that Americans should hold dear. An attack on any American group is a threat to the pluralistic fabric of our nation.

Fourth, despite the efforts of many elected officials and law-enforcement agencies to keep us safe, more needs to be done — including enhanced information-gathering, tougher prosecution and sentencing and increased public education — to respond to anti-Semitic attacks in our communities.

One model for community engagement is what happened in Billings, Mont., in 1993, after a year of racist and anti-Semitic incitements that culminated with the throwing of a brick through the bedroom window of a young child in a Jewish home that had a menorah on display. The reaction of Billings was swift and decisive. Under the leadership of the police chief and a newspaper editor, paper cutouts of a menorah were made widely available. Thousands of households in Billings put them in their windows. The message was clear: Anti-Semitism and racism had no place there.

In a survey of American Jews by the American Jewish Committee, released in October, 31 percent of respondents said that they had taken steps to hide their Jewish identity in public, while 25 percent said they now avoided Jewish sites. And this survey was conducted before the recent attacks in Jersey City and Monsey.

This is unacceptable. It is not our America. We call on all Americans of good will to ask ourselves how each of us can defend our inclusive vision for this country.

Nita Lowey is a United States congresswoman whose district includes Monsey. David Harris is the chief executive of American Jewish Committee.