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In the 19th century, it was incredibly dangerous to be a child.

As of 1900, about 18 percent, or nearly one in five, American children died before their fifth birthday. The most common causes were infectious diseases — pneumonia, diphtheria, dysentery, measles, and other illnesses ran rampant through households, and children were especially at risk.

Cities, in particular, were “cauldrons of infection,” Samuel Preston, a demographer and co-author of the book Fatal Years: Child Mortality in Late Nineteenth-Century America, told me. But around the country, communicable diseases were “rites of passage of childhood, some of them far worse than others, but all of them causing serious morbidity, and a lot of them causing death,” said Howard Markel, a historian of medicine who has studied epidemics.

Today, by contrast, less than 1 percent of children die before the age of 5, and until recently, once-common childhood diseases like measles were essentially unheard of in the US. What changed?

Better sanitation and understanding of germ theory are part of the story, but one key factor that’s transformed American childhood over the past century is the widespread adoption of vaccines. Today, children in the US are routinely vaccinated against measles, mumps, rubella, diphtheria, polio, some types of pneumonia and meningitis, and more. Other vaccines, including for typhoid, are in use around the world.

This public health victory has saved hundreds of millions of lives and prevented billions of cases of disease. On a list of “the 10 greatest hits of medicine,” Markel said, at least nine would be vaccines.

That message has been getting lost lately, thanks to a rise in anti-vaccine sentiment around the US and the world, exemplified most recently by the confirmation of Robert F. Kennedy Jr. one of the country’s most prominent vaccine skeptics, as the secretary of Health and Human Services. Just weeks into Kennedy’s tenure, a measles outbreak has sickened more than 100 people and killed an unvaccinated child........

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