Diseased Illegal Immigrants Aren't "Invading" the United States

Alex Nowrasteh and Krit Chanwong

President Trump issued an executive order (EO) on January 20 titled “Guaranteeing the States Protection Against Invasion,” wherein he declares that illegal immigrants are invading the United States and he’s responding by suspending asylum along the southern border, restricting immigration more broadly, and activating other government powers. Trump’s declaration of invasion gives this justification for such a drastic order:

The sheer number of aliens entering the United States has overwhelmed the system and rendered many of the INA’s [Immigration and Nationality Act] provisions ineffective, including those previously described that are intended to prevent aliens posing threats to public health, safety, and national security from entering the United States. As a result, millions of aliens who potentially pose significant threats to health, safety, and national security have moved into communities nationwide.

My research at the Cato Institute on crime and terrorism committed by illegal immigrants conclusively shows that they commit less crime than native-born Americans and have murdered zero people in domestic attacks since 1975. This post fills in the gap in Trump’s invasion declaration by analyzing how immigrants affect disease in the US. We find no statistically significant relationship between the size of the immigrant population, the illegal immigrant population, or the legal immigrant population and the spread of serious communicable diseases on the state level during the 2021–2023 period when America was supposedly being invaded by diseased immigrants. 

In other words, no reasonable disease prevention justification exists for the Trump administration’s declaration of an invasion.

Background

The US government should stop immigrants who threaten the health, safety, and national security of Americans from entering the United States. The first Trump administration closed the border during the pandemic by invoking Title 42, a public health law that empowers the government to prevent all people from crossing the border by halting legal entries and immediately removing unlawful border crossers to stop the spread of communicable diseases to the United States from abroad. 

Before the COVID-19 pandemic during Trump’s first term, some of his immigration advisors searched in vain for a disease carried by migrants to justify invoking Title 42. The pandemic justified such extraordinary measures that the Biden administration kept in place until May 2023.

When Title 42 was in effect from March 2020 to May 2023, Border Patrol encountered about 8.5 million illegal border crossers along the US-Mexico border. These aren’t individual crossers but the number of crossings encountered by Border Patrol agents. During that time, 46 percent of those were returned to Mexico under Title 42 authority—or almost 3.9 million. The major effect of Title 42 was to make the government seem tougher on immigration enforcement by dumping people immediately back over the Mexican border. But it also changed the incentives for illegal immigrants and caused more of them to try to cross more often. The result was higher numbers.

Prior to Title 42, illegal border crossers were detained for a period and sometimes charged with minor crimes like crossing the border illegally. Detention before Title 42 was a punishment and a deterrent to crossing again. After all, illegal immigrants come to the US for economic opportunity and often pay thousands of dollars or more in smuggling fees for the chance to work for wages 3–15 times higher than in their home countries. Every minute they spend in government detention imposes a significant cost on them, so detention is quite the deterrent to crossing illegally. Title 42 changed those incentives, lowering the cost of trying to cross the border repeatedly—and that’s what happened.

However, the political incentives were different. More illegal immigration caused chaos that voters despised, so politicians despised it too. The reaction to chaos was to attempt to reduce it through more enforcement, which often doesn’t work and can even make the problem worse. Title 42 was a textbook example of this. 

Initially justified for public health reasons, justifications morphed after the end of the pandemic from reducing the harm of COVID to reducing illegal immigration. By the time Biden finally ended the COVID-19 emergency and Title 42, other politicians who correctly and vociferously criticized him and others for the long duration and use of extraordinary anti-pandemic powers began to sing a different tune.

Senator Ted Cruz (R‑TX) called the end of Title 42 a “travesty.” Republican Senators Lindsey Graham (SC), Mitch McConnell (KY), John Cornyn (TX), Marsha Blackburn (TN), Chuck Grassley (IA), Mike Lee (UT), Josh Hawley (MO), Tom Cotton (AR), John Kennedy (LA), and Thom Tillis (NC) issued a press release that said:

We shudder to think about how much worse the situation at the border would have been over the past three years had it not been for the deterrent effect of Title 42. Over the past three years, the Title 42 order has been a lifeline to the men and women of Border Patrol, who have been working heroically 24 hours a day to secure our southern border amid the worst border crisis in our lifetimes. Even with Title 42 in place, illegal crossings at the border have been at all-time highs … Our border remains under assault. The resources of US Customs and Border Protection (CBP) remain under tremendous strain. The introduction of 13,000 encounters every day to this crisis would be the equivalent of throwing gasoline on an already raging fire.

Title 42 morphed from an anti-disease measure into a border enforcement tool. Law enforcement fell for this, too, and the concern about removing Title 42 was palpable in the Department of Homeland Security during this time. That concern was unsurprising because their job was to enforce laws rather than think about the incentives for lawbreaking, but it turned out to be unwarranted. The number of border encounters immediately fell after the end of Title 42 and then returned to trend under the slowing labor market, which reduced the demand for illegal immigrants in early 2024, causing the number of illegal border crossers to fall.

This time, the Trump administration is not relying on Title 42 to close the border but is instead arguing that illegal immigrants are invading the United States because they could be spreading disease and danger by being insufficiently vetted. There’s no good safety or security reason to declare an invasion. Indeed, if it were a real invasion then the administration wouldn’t have had to declare it exists by pointing to insufficient vetting of illegal immigrants. We’d all know it was occurring. The imposition of such a serious declaration of invasion is fact-dependent, and the facts do not agree with those who support closing the border to stop the spread of disease.

Method of Analysis

If immigrants spread disease, then there should be a high correlation between disease incidence and the number of immigrants in an area. To investigate this hypothesis, we ran several two-way fixed effects regressions with state and year fixed effects and standard errors clustered at the state level. In our regression models, the immigrant share of the population is the independent (explanatory) variable and disease rates were the dependent (outcome variable).

We gathered state-level disease data from the CDC’s National Notifiable Disease Surveillance System (NNDSS), which tracks the occurrence of approximately 120 diseases and conditions. A notifiable disease is “A disease that, when diagnosed, requires health providers (usually by law) to report to state or local public health officials. Notifiable diseases are of public interest by reason of their contagiousness, severity, or frequency.” We excluded COVID-19 from the analysis. We then plotted the disease incidence rates with estimates of the immigrant population retrieved from the American Community Survey (ACS). We imputed the number of illegal immigrants using a modified Gunadi method to estimate the state-level legal and illegal immigrant population shares.

In addition to analyzing all disease incidence, we also focused on two serious diseases that may be widespread in immigrant communities and would be most likely to justify a public health emergency: measles and tuberculosis. Data for measles were also taken from the NNDSS, while data for tuberculosis prevalence was retrieved from the Online Tuberculosis Information System (OTIS), which is also retrievable through CDC Wonder. We focused only on the 2021–2023 period where data are available. In some cases, we can only analyze data during 2021 and 2022. We focused on these periods because they cover parts of the Biden administration that Trump’s invasion declaration is responding to.

Results

There is no relationship between monitored diseases and the share of all immigrants, illegal immigrants, or legal immigrants at the state level during the 2021–2022 period. Figure 1 plots the incidence of all notifiable diseases per 100,000 against the share of all immigrants on the state level. Figures 2 and 3 are the same analysis for illegal immigrant and legal immigrant shares of the population, respectively. They all show no relationship. Table 1 confirms the visual analysis: there is no statistically significant relationship between notifiable disease rates and the all immigrant, illegal immigrant, or legal immigrant shares of the population.

Figures 4–6 plot the incidence of measles per 100,000 residents against the share of all immigrants, illegal immigrants, and legal immigrants on the state level during 2021–2022. There is no visual correlation between measles incidence rates and any measure of the foreign-born population during those two years. 

Table 2 shows the output from our two-way fixed effects model, where we find no statistically significant correlations between any measure of immigrant shares of the population and the incidence of measles.

Unlike overall disease incidence and measles, there is a visually weak positive relationship between tuberculosis and all immigrant, illegal immigrant, and legal immigrant shares of the population (Figures 7–9). Interestingly, the two-way fixed effects regression results show that the positive correlation between tuberculosis and illegal immigrants and all immigrants is not statistically significant (Table 3). They also show that, with year and state-fixed effects, a one percent increase in legal immigration is correlated with a –0.542 change in tuberculosis prevalence per 100,000.

Discussion

There were no statistically significant relationships between immigrant population shares and notifiable disease incidence rates on the state level. But the figures for tuberculosis warrant a deeper dive into that disease. There were 9,633 cases of tuberculosis reported in the United States in 2023, similar to the same number in 2013. In the intervening decade, the number gradually declined and then cratered in 2020 and 2021 during the pandemic when the health system was severely interrupted by COVID and immigration to the US slowed.

Between two-thirds and three-fourths of all tuberculosis cases are among the foreign-born, or 7,299 in 2023, a rate about 18.75 times higher for immigrants than native-born Americans in 2023. However, the number of annual tuberculosis cases among immigrants has remained about the same per year since 1993, when there were 7,414 cases. In the intervening 30 years, the foreign-born population more than doubled, and the number of tuberculosis cases stayed about the same, resulting in a decline in the rate of tuberculosis among the foreign-born from 34.1 per hundred thousand immigrants in 1993 to 15 in 2023. The correlation between the rate of tuberculosis among immigrants and native-born Americans over the entire period is strong because they were both falling. The real decline of tuberculosis in the US occurred among the native-born population, where cases fell from 25,102 in 1993 to 9,633 in 2023, or from 7.4 per 100,000 to 0.8 per 100,000. A decline in the rate of tuberculosis is evidence that that disease is not invading the United States.

Those individuals should be treated for tuberculosis, but there isn’t statistically significant evidence of them spreading tuberculosis to native-born Americans or other residents. The United States has long screened immigrants for serious communicable diseases and should continue to do so. However, Trump’s declaration of invasion depends on several facts about crime, national security, and disease that aren’t true. 

A new pathogen has not emerged overseas and there is no evidence of a known serious disease like measles or tuberculosis being spread by immigrants in the United States. The claims of illegal immigrants spreading disease are just as untrue as claims of them spreading crime and terrorism.