Smallpox is a horrendous disease that killed more than 300 million people in the 20th century alone, and millions more before that. But it’s a scourge of the past — the only human virus to have been eradicated.

Remembering that, you can appreciate the stupidity of the statement on Fox News — by an ex-ICE agent — that the migrant caravan of 4,000 men, women, and children mainly from Central America is going to bring smallpox to America.

“They are coming in with diseases such as smallpox, leprosy, and TB that are going to infect our people in the United States,” the former agent, David Ward, said this week.

There is no smallpox in circulation anymore. That’s been true since 1980, when a major global vaccine effort wiped the virus from the planet. The risk of leprosy — now called Hansen’s disease — being imported from Latin America is similarly remote. And while some foreign-born people do have higher rates of TB, the Centers for Disease Control and Prevention (CDC) screens for TB in people moving to the US.

This particular kind of xenophobic fear-mongering, which Donald Trump spread as a presidential candidate, is now surfacing again as we approach the midterms, in an apparent ploy to rile up the conservative base.

But the bottom line on public health is this: Travelers and visitors do spark outbreaks in the US from time to time. But they tend not to be refugees from Honduras, but Americans who get sick overseas, and then come back to the US and infect people in their communities who have refused vaccines for themselves or their kids.

Many infectious disease outbreaks in the US are made worse by vaccine refusal

One of the largest outbreaks of measles in recent US history was sparked by an American missionary, who brought the virus back from the Philippines to his unvaccinated Ohio Amish community. Just before that, in the first half of 2014, 97 percent of the 288 measles cases in the US were caused by travelers bringing the disease in from other countries — mainly returning to the US from the Philippines.

Another big measles outbreak, among Minnesota-born Somalis, grew out of control because anti-vaxxers stoked autism fears in that community and people stopped vaccinating their kids. The people most affected by that outbreak were American-Somali children.

“Had the anti-vaccine movement not been successful here, these would have been non-issues,” said Peter Hotez, director of the Texas Children’s Hospital Center for Vaccine Development at Baylor College of Medicine.

People coming from these countries often have better rates of vaccine coverage than Americans. Brazil has 100 percent coverage for the measles-mumps-rubella (MMR) vaccine, for example. El Salvador has 94 percent coverage, Honduras has 88 percent coverage, and Mexico has 98 percent coverage. In the US, meanwhile, the MMR rate is 91 percent.

As my colleagues at the Verge point out, while the CDC acknowledges the risk of migrants spreading disease in the US, there’s no evidence of large outbreaks at the border,

The idea that leprosy is being imported to the US from Latin America is also absurd. “We have leprosy transmission in Texas and Louisiana from armadillos,” Hotez said. “We have registered in the US around 200 people registered (with leprosy).” Mexico has a comparable burden of Leprosy — about 500 cases — while El Salvador has only six, Honduras has nine, and Nicaragua has 21. “So leprosy is a non-factor.”

This is xenophobia

So if there’s little truth to the disease-carrying migrants story, then what’s really behind it? The occasional ugly human tendency toward xenophobia.

There’s a long history of accusing foreigners of harboring and spreading disease. It’s one way humans’ instinctual distrust of “other” people manifests itself. Adam Rogers had a great rundown of examples of so-called diseased migrants in Wired:

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. (It’s only fair to point out that immigrants did indeed bring smallpox to North America — white Europeans, beginning with Christopher Colombus’ crew.)

We saw the same xenophobia during Ebola, when countries, including Canada, issued nonsensical visa bans for people from Ebola-stricken West Africa. And we could see more of it rear its head as the political races heat up in the next few days. So just remember that human bias and the lack of evidence support the fear.