By that fall, Trump’s views on the value of public health spending had come to dominate political discourse. Zika, the mosquito-borne virus that causes grave birth defects, began to spread within Puerto Rico in December 2015, and the next month a baby was born in Hawaii with Zika-related microcephaly, the first signs of a multiyear outbreak that’s produced more than 43,000 confirmed diagnoses in the United States and its territories. In February 2016, President Obama sent an emergency request to Congress for $1.8 billion in funding to help fight the disease. The Republican-led Congress cut the request by $700 million, however, and then took 233 days to approve it. “That delay was driven, in no small part, by a baseless view that Zika was a disease being brought to our country by immigrants,” Klain said. “As a result, you had local transmission of Zika in Florida in 2016. You had for the first time in our country’s history the Centers for Disease Control issuing a travel advisory against part of the continental United States.”

Two weeks before last year’s midterm elections, as a migrant caravan made its way through Central America toward the U.S. border, the idea of immigrants posing a health risk to the American populace emerged again. “It’s a health issue, too, because we don’t know what people have coming in here,” Fox News host Laura Ingraham said on October 23. Two days later, a guest on her program raised the issue as well, saying, “We have mass amounts of people coming into our country with disease.” Fox & Friends next picked up the theme of disease-bearing immigrant hordes, followed by a Lou Dobbs guest on the Fox Business Network who linked “the continued invasion of this country” with “diseases spreading across the country that are causing polio-like paralysis of our children.” The most explicit outburst of this rolling moral panic probably came from David Ward, a former U.S. Immigration and Customs Enforcement agent who claimed on the Fox News show Your World with Neil Cavuto, “We have these individuals coming from all over the world that have some of the most extreme medical care in the world, and they are coming in with diseases such as smallpox and leprosy and TB that are going to infect our people in the United States.”

These assertions are either flatly false (smallpox was declared eradicated in 1980) or grossly inflated (roughly 34,000 cases of leprosy, now called Hansen’s disease, occurred in the Americas in 2015, 94 percent of which were clustered in Brazil—not the source of the caravan). Ironically, the caravan members might well have posed less of a risk to the American public than other U.S. residents do, because the countries from which they were fleeing have higher routine vaccination rates than those recorded in many parts of the United States. But even after the midterms, Fox News hosts and guests continued to promote the idea that immigrants to the United States were bringing with them the seeds of the next great pandemic. At the end of November, Fox & Friends First co-host Jillian Mele announced the network had “exclusively” found “serious health risks being carried by some of those migrants”—risks that, according to correspondent Griff Jenkins, included lice, chicken pox, and the common cold. And in December, in the infamous White House press briefing in which he promised to “take the mantle” of shutting down the government, Trump reemphasized the now-familiar point: “People with tremendous medical difficulty and medical problems are pouring in, and in many cases it’s contagious. They’re pouring into our country. We have to have border security.”

Nationalist politics has led to a massive measles outbreak in Europe. 2016: 5,273 cases 2018: 82,596 cases Source: World Health Organization

The claim that immigrants are intrinsically dangerous long antedates Trump’s rise to power. Indeed, it was the justification for U.S. immigration restrictions for more than a century. “Whether we’re talking about the Irish during the great cholera epidemics of the antebellum period, or polio and the Italians, or tuberculosis from the Jews, or bubonic plague and the Chinese, it’s a recurring concern that Americans have,” said Alan Kraut, a professor of history at American University and the author of Silent Travelers: Germs, Genes, and the “Immigrant Menace. “The popular theme is they are ‘dirty,’ and a challenge to the public self.”