You know what the great thing about smallpox is? It doesn’t exist anymore.

Nearly 40 years ago, it was officially eradicated from the face of the Earth – the first, and so far only, disease ever to be declared as such.

So it would be pretty difficult for anybody in the migrant “caravan” currently traveling towards the US from Central America to bring the smallpox virus with them. Yet that’s what a Fox News guest told viewers in a segment this week: that these people, many of whom are seeking asylum, pose a public safety hazard, and will infect “our people” with smallpox, leprosy, and tuberculosis (TB).

Needless to say, this is not true.

Leprosy, or Hansen’s disease, is an illness that terrified ancient cultures. Unlike smallpox, it’s still with us today, and there’s no effective vaccine against it.

But luckily, leprosy is rare, easy to treat, and quite hard to transmit – a “wimp of a pathogen”, according to specialists. A full 95 percent of people have a natural immunity against it – and for those without, even more good news: there’s a cure.

And while it’s true that the rates of TB are higher in people born outside the US, the CDC screens for the disease in inbound migrants. So there’s no need to worry about that either.

Neither should we blame immigrants for the recent cases of acute flaccid myelitis (AFM), the mysterious polio-like disease that’s turned up in more than 20 states so far – even though that’s exactly what a guest on Fox show Lou Dobbs Tonight said this week.

In fact, we know hardly anything about AFM. We don’t know what causes it or how it’s transmitted. We can’t say people from other countries are spreading it – we have no idea what’s spreading it.

The USA is a technologically advanced nation in a vastly interconnected world. Contagious diseases aren’t being brought in by people seeking a better life – they’re already here.

“There’s the misconception that somehow the United States is protected from those diseases and that’s simply not true,” Peter Hotez, dean of the National School of Tropical Medicine at Baylor College of Medicine in Houston told The Verge. “The major factors promoting these diseases are poverty and urbanization and climate change. The idea that a few thousand immigrants are going to create a bump in that just defies the numbers.”

If you’re truly worried about contracting TB, or measles, or the flu, there’s an easy solution: vaccination. And as many people have pointed out, the US anti-vaxxer movement is more of a threat than people coming from El Salvador (up to 93 percent vaccine coverage), Honduras (likewise 93 percent) or Guatemala (up to 98 percent vaccine coverage).

So if there’s no threat from migrants, what’s behind all the recent rhetoric?

Stoking fears that immigrants will bring diseases has a long and dark history. As Adam Rogers at Wired explains: “People called cholera the ‘Irish disease’ in the 1830s and tuberculosis the ‘Jewish disease’ in the 1890s. And the 1918 flu epidemic coincided with a giant wave of immigration into the US, prompting all sorts of fears of the disease’s further spread.”

Despite their frequent scientific blind spots, politicians certainly know one thing: when people are scared of diseases, they get irrational and xenophobic – and they vote for right-wing, authoritarian parties.

In light of the upcoming elections, it’s important to see this for what it is: fact-free political rhetoric – based not on science or evidence, but simple xenophobia.