By Aaron Kliegman\u00a0 Antisemitism is a virus that mutates with time to conform to what is socially, culturally, and intellectually acceptable. In the Middle Ages, Jews were hated and persecuted because of their religion. Then religious persecution became taboo during the Enlightenment. So, antisemitism mutated. In the 19th and 20th centuries, Jews were hated and persecuted because of their race. Then racial persecution became taboo following the horrors of the Holocaust, along with the civil rights movement in the United States. So, antisemitism mutated. Since the mid-20th century, Jews have been hated and persecuted because of their nation-state, Israel. This latest mutation especially attacks Zionism, the millennia-long desire and resolve of Jews to re-establish, and then to preserve, their national home in the land of Israel. So far, antisemitism hasn\u2019t had to mutate again, enjoying the freedom to infect people, institutions, and societies in its anti-Zionist form. All three types of antisemitism exist today, often overlapping and feeding off each other. Indeed, Islamists, neo-Nazis, and radical progressives use each mutation to fuel wide-ranging extremist movements and ideologies that have one common enemy: the Jews. From opening fire inside synagogues to spray-painting swastikas on Jewish gravestones, from berating Jews with verbal abuse to demonizing Israel as an illegitimate state guilty of apartheid, the scourge of antisemitism is devastating families and communities. Humanity has yet to find a vaccine for civilization\u2019s oldest virus. Like any problem, we must define antisemitism in order to confront it. That\u2019s just common sense. Otherwise, any effort to fight this virus will be directionless, based on subjective gut feelings rather than objective standards. But antisemitism is an especially serious and complex problem: The definition must be comprehensive enough to recognize and account for all three aforementioned mutations but clear and specific enough to be practically useful. Fortunately, such a definition already exists. And we can thank the International Holocaust Remembrance Alliance (IHRA) for providing it. IHRA is an intergovernmental organization that unites governments and experts to combat antisemitism and to research, educate the world about, and promote remembrance of the Holocaust. In 2016, IHRA, then comprised of 31 countries (now 34), formally adopted a non-legally binding \u201cworking definition of antisemitism.\u201d According to the definition, antisemitism \u201cis a certain perception of Jews, which may be expressed as hatred toward Jews. Rhetorical and physical manifestations of antisemitism are directed toward Jewish or non-Jewish individuals and\/or their property, toward Jewish community institutions and religious facilities.\u201d IHRA strengthens its definition by providing 11 specific, contemporary examples of antisemitism in public life, the media, schools, the workplace, and in the religious sphere. Critically, the examples include denial of the Holocaust and also newer forms of antisemitism targeting Israel \u2014 such as demonizing the Jewish state, denying its right to exist, and holding it to standards not expected of any other democratic state. Since 2016, dozens of countries and hundreds of local authorities, other public institutions, universities, and even sports teams have adopted IHRA\u2019s definition. These entities recognize that IHRA\u2019s definition is a practical tool. It creates a baseline to use for teaching, training, legislating, and, most importantly, combatting antisemitism in concrete ways. As United Nations Secretary-General Antonio Guterres said in 2018, the definition \u201ccan serve as a basis for law enforcement, as well as preventive policies.\u201d And, indeed, it has. Consider the United Kingdom College of Policing, which uses the definition in its Hate Crime Operational Guidance for police training. Or consider the UK\u2019s Judicial College, which includes the definition in its guidance to judges. Various entities across Europe \u2014 from police in Germany to Holocaust memorials in Austria \u2014 use the definition in their training, understanding it will help identify, prosecute, and punish antisemitic crimes. The definition is also essential for governmental agencies and non-governmental organizations to collect and record data on antisemitic incidents. Unfortunately, some people oppose IHRA\u2019s definition, claiming it infringes on free speech by prohibiting criticism of Israel. They conveniently ignore how the definition states that \u201ccriticism of Israel similar to that leveled against any other country cannot be regarded as antisemitic.\u201d They also ignore the fact that antisemitic speech is protected in the US, unless it incites imminent crime. Nonetheless, in January, several far-left Jewish groups released a joint statement attacking IHRA\u2019s definition and calling for it to be replaced. And then late last month, some 200 activists and academics unveiled a new and dangerous definition of antisemitism explicitly meant to undermine and replace IHRA\u2019s version. According to this definition, called the Jerusalem Declaration on Antisemitism, anti-Zionism is not antisemitic. Nor is the Boycott, Divestment, and Sanctions movement (BDS) against Israel. Nor is applying double standards to the world\u2019s only Jewish state. Nor is demonizing and delegitimizing the existence of the Jewish state as a racist endeavor akin to Nazi Germany. What\u2019s so remarkable about this definition is that its authors seem far more concerned with protecting people accused of antisemitism than with protecting Jews victimized by antisemitism. Indeed, they\u2019re not actually fighting antisemitism; they\u2019re fighting those who are fighting antisemitism.