Former President Donald Trump left the White House amid a public health emergency that his own messaging helped exacerbate. Now, as he campaigns to retake the presidency, he is once again politicizing long-held best practices for stopping infectious diseases.

With Covid-19 again on the upswing and amid an ongoing measles outbreak, the former president is taking aim at school vaccine requirements from the campaign stump, promising to defund the districts that mandate students receive certain vaccines.

The real danger in Trump’s words is not any actual loss of education funding — that threat is toothless, experts say — but the continued normalizing of anti-vaccine attitudes.

People are already losing trust in vaccines: Only 40 percent of Americans believe it is extremely important for parents to get their children vaccinated, down from 64 percent in 2001. It is perhaps the most worrying trend in public health right now.

We have the tools to stop many infectious diseases — if we take advantage of them. Trump’s words are making it less likely that people will.

Trump had a famously complicated relationship with Covid-19 vaccines during the pandemic. While he officially endorsed them, and his administration played a critical role in their development, he simultaneously indulged vaccine hesitancy on the right, where a tangle of conspiracy theories cast the shots as a part of a sinister public health agenda. Prior to being elected, he entertained theories (advanced by his frenemy in the 2024 presidential race, RFK Jr.) about a link between vaccines and autism. Recently leaked footage suggests he still privately shares those views.

Now, Trump is promising to make it a priority to take action against schools with vaccine or mask mandates and those that “teach” critical race theory.

“On day one, I will sign a new executive order to cut federal funding for any school pushing critical race theory, and I will not give one penny to any school that has a vaccine mandate or a mask mandate,” he told the Christian Faith and Freedom Coalition, a conservative Christian group, in July.

When I heard Trump’s comments, I immediately thought: That can’t be right.

I’ve covered education policy over the years, and I knew the federal government provides relatively little of the money — 10 percent or so — for public education in this country. Most of it comes from state and local governments.

And Trump can’t just do whatever he wants with federal education dollars, either.

The bulk of federal funding is authorized by the Elementary and Secondary Education Act, the federal law that has set most of federal education policy since the 1960s (with several updates over the decades). That spending has fairly specific strings attached, leaving the federal government with a very limited say over how state and local jurisdictions spend their much larger share of school funding.

That would limit Trump’s ability to conjure up some justification for withholding funding from schools if he tries to follow through on his threat, Kevin Carey, who leads education policy at the New America Foundation and is an occasional Vox contributor, told me. (Desegregation is a rare example of Washington, DC, using the withholding of funding.)

But Carey worries Trump could still influence local school decisions, especially if he’s elected and even if he can’t actually block funding, because he could still use his bully pulpit to make school administrators fear litigation or more subtle retaliations if they defy the federal government’s wishes.

That kind of softer influence is where the real risk lies. Trump’s words reach a lot of people. So much so that there is a small library’s worth of research on how Trump affected the Covid-19 information ecosystem and the public’s attitudes toward vaccines.

One meta-review of 1 million news articles about the pandemic found that nearly 40 percent of them featured Trump and one of the false claims attributed to him. Coverage of specific subjects, such as miracle cures or the deep state, paralleled whatever fixation Trump had at a given time or the enemies he was railing against.

Several studies linked messages that featured Trump’s endorsement of Covid-19 vaccines with an increase in actual vaccinations — another measure of his influence. On the flip side, being a Trump voter was associated with a lower likelihood of getting vaccinated, which reflects festering conspiracy theories and anti-public health sentiments in Trump’s base.

One Pew survey found that people who trusted Trump the most for pandemic information were the least likely to be vaccinated. This may help to explain why the Trump-voting parts of the US saw more deaths adjusted for population during the pandemic.

When Trump starts to badmouth not just Covid-19 vaccines, but also routine childhood vaccinations that have been around for decades and proven their efficacy in preventing deadly diseases, he is adding fuel to a dangerous fire.

Measles, mumps, and rubella vaccinations — the common “MMR” shot that is required by almost every state — have been dipping since the 2019-2020 school year. They are now below the 95-percent coverage threshold that experts consider to be necessary for herd immunity that would stop the viruses’ wide transmission. The number of exemptions granted to families who do not wish to vaccinate their children has reached record levels. States, mostly those with conservative politics, have been proposing further loosening of the vaccine requirements that currently exist.

Meanwhile, measles cases in the US matched their 2023 total over just the first few months of 2024. A local outbreak in Oregon has seen nearly two dozen cases since June; at least two people have been hospitalized.

A disease that was once effectively eradicated in the US — and which school mandates helped to stamp out — is mounting a comeback.

Donald Trump could choose to wield his tremendous influence to try to restore people’s faith in vital public health measures. He did it, if half-heartedly, during the pandemic and it had the desired effect. Instead, he’s stoking doubts about the value of vaccines, and courting the dangers vaccine hesitancy brings.

This story originally appeared in Today, Explained, Vox’s flagship daily newsletter. Sign up here for future editions.