The mission of the federal Department of Health and Human Services is a simple one: “to enhance and protect the health and well-being of all Americans.”

One of the most effective ways to protect the health of Americans has proven to be vaccinations against infectious diseases. Before the introduction of the polio vaccine in the mid-1950s, tens of thousands of people were afflicted. In 1952, nearly 3,200 people died. By 1960, there were a few hundred cases, and 90 deaths. These days, there are usually no reported cases in a year.

Another example. For the first five years of the 1940s, there were a million cases of whooping cough. When a vaccine was introduced at the end of that decade, the number of cases fell to 15,000 by 1960 — and 1,700 by 1980. But then the number of cases started to go back up, as immunization rates fell. Why? In part because a British researcher fraudulently linked vaccines to autism in 1998, a claim that was later retracted. But the idea took hold, spooking parents and causing immunization rates to drop.

During a town hall on CNN this week, HHS Secretary Tom Price was asked if he thought that immunizations against infectious diseases should be mandatory. His answer? It’s up to the states.

“I believe it’s a perfectly appropriate role for the government — this happens by and large at the state government level, because they’re the ones that have the public health responsibility — to determine whether or not immunizations are required for a community population,” Price said, “whether it’s growing kids or the like or whether there’s an outbreak of a particular infectious disease, whether or not an immunization ought to be required or be able to be utilized.”

While states are already in charge of immunization policy — and they require certain vaccines — Price’s less-than-rousing endorsement of mandatory immunization threatens his organization’s mission.

The reason? “Herd immunity.” Immunizations protect individuals from becoming sick, sure, but if you immunize enough of the population, you can prevent rampant outbreaks from getting a toehold in the first place.

We can illustrate this easily, using this animated image as inspiration. Adjust the immunization rates on the chart below and see what happens when you introduce a sick person into the population. (For those curious, in our simulation an unvaccinated person stands a 50 percent chance of becoming sick if they’re adjacent to an ill person.)

Part of the problem with relying on states to police themselves without national guidance is that if a state decided to drop the immunization requirement, things can get bleak in a hurry. You probably notice the “different vaccination rates” toggle above. If the population on the right has lower vaccination rates than the one on the left, an outbreak can originate in that right state and spread over to the left easily. If vaccination rates were equally high on both sides, that’s much less of an issue.

Why did Price not simply tell Wolf Blitzer on national television that immunization was critical? Perhaps in part because of the position of his boss, President Trump. Trump has repeatedly questioned the safety of vaccines, parroting the even-then discredited link to autism.

As president, he’s apparently asked Robert Kennedy Jr., another vaccine skeptic, to head a “review” of vaccine safety.

In 1955, 467 people died of whooping cough. In 1985, four did. In 2005, the number was back up to 31. The problem with loosening vaccination rules is that the population broadly becomes more susceptible to illness. And when that happens, people die.