If you stand on platform 17 of Berlin-Grunewald railway station, the horror of antisemitism never leaves you. I stood there with my friend, silently trying to fathom what befell his grandmother’s relatives as they were deported to Latvia, where they would end up gunned down in forests.

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I write this in a cafe full of laughing customers, cheesy music blaring in the background. A platform full of families with screaming children being squeezed together in unbearable conditions, on their way to be massacred and buried anonymously in mass graves, is a nightmarish parallel universe beyond my comprehension. But it happened. And it wasn’t some mid-20th century aberration that came out of nowhere, a bafflingly horrific episode in human history resulting from sudden mass insanity. This was the culmination of hundreds of years of antisemitism: pogroms, blood libel, scapegoating.

Antisemitism is currently being discussed in the context of the Labour leadership contest, of which more shortly. But suffice to say that, although the sole attempt in human history to exterminate an entire people by industrialised means forced Europeans to confront pandemic antisemitism, this cancerous hatred remains.

It can be overt. Take the neo-Nazi Golden Dawn in Greece, feeding off the despair of austerity; take the antisemitic Jobbik party in Hungary, which one in five Hungarians voted for last year; take the foul intolerance of murderous Islamic fundamentalism. But it is not simply the preserve of neo-Nazi skinheads; it can be subtle too, and it finds expression not just on the right but on the left. All cases need to be confronted for what they are.

Corbyn could not have known the background of every individual at every rally he has attended, but some were antisemitic

The Labour leadership frontrunner, Jeremy Corbyn, has been a long-term supporter of the Palestinian justice movement. He could not possibly have known the personal backgrounds of every individual who has joined him at the many rallies he has attended over the years. Some of these people were antisemitic. And while the vast majority of people involved in the movement are – like myself – driven by a passionate support for self-determination, there is a minority that indulges antisemitic tropes. These ideas have to be defeated.

Yes, supporters of Palestinian justice get unfair criticisms. “Why do you focus on the plight of the Palestinian people rather than, say, the horrors committed by Islamic State or the suffering of the Kurds?” some ask. (Leftwingers were protesting about the plight of the Kurds outside the Iraqi embassy when our western governments were arming Saddam Hussein.)

Israel – unlike, say, Isis – is backed by democratic western governments whose foreign policy we can influence. And the Israel-Palestine question, an intractable conflict stretching back decades, has long been the key foreign policy issue for supporters of both Israel and Palestine.

Some ardent supporters of the Israeli government oppose all critics of Israeli policy and accuse them of antisemitism (or, if those critics are Jewish, of being “self-hating Jews”). I’ve encountered it myself. At a recent wedding a former Times journalist I’d never met apprehended me and accused me of antisemitism, threatening to punch me.

But some passionate supporters of Palestinian justice deny antisemitism exists and regard all accusations of it as an attempt to shut down criticism of Israel. While they would never dream of denying the existence of racism against, say, black people or Muslims, they treat antisemitism as a political device constructed by militant supporters of Israeli occupation. And in doing so, they fail to properly scrutinise it within their own ranks; there are those who are soft on it.

I have challenged dodgy pronouncements from people who profess to advocate Palestinian justice.Jewish people are sometimes told that antisemitism is caused by Israel’s actions, for example. These are the same people who would never dream of victim-blaming members of other minorities, or claim that anybody was at fault other than the bigot themselves. Others play linguistic games: how can it be antisemitism, they say, when Palestinians are also “Semites” – members of a group of people originally of the ancient Middle East that includes Jews and Arabs – even though “antisemitism” has meant “anti-Jewish hatred” for generations. (This is like saying, “I’m not homophobic because I’m not scared of gays.”)

There are those who imply that Jewish people are somehow synonymous with the Israeli government (a slur echoed by some uncritical cheerleaders of Israeli state policy). And some use terms like “Jewish lobby”, a classic antisemitic trope suggesting there is an organised Jewish cabal exercising behind-the-scenes influence worldwide. And so on.

Antisemitism is too serious to become a convenient means to undermine political opponents. It is a menace: not just in its overt forms, but in subtler, pernicious forms too. There’s no excuse for the left to downplay it, or to pretend it doesn’t exist within its own ranks. Rather than being defensive, the left should seize any opportunity to confront the cancer of antisemitism and eradicate it for good.