One-year-old River Jacobs receives an MMR vaccine at a 2025 vaccine clinic in Lubbock, Texas. | Jan Sonnenmair/Getty ImagesThis story appeared in Today, Explained, a daily newsletter that helps you understand the most compelling news and stories of the day. Subscribe here.America formally eradicated measles in the year 2000. But like many other remnants of the 1990s — flip phones, CDs, baggy pants — measles is now regrettably and undeniably back. More than 2,300 Americans have come down with measles so far this year — the highest number since 1992. New data from the Centers for Disease Control and Prevention shows that there’ve been more cases in the country in the last two years than in the 24 years prior. Many of the cases trace back to large outbreaks in South Carolina, Utah, Virginia, and Pennsylvania, some of which have ended. But the underlying problem is both much broader and much trickier to address: As more parents pass on routine childhood vaccinations for their kids, these once-eradicated viruses have more chances to spread.For measles, which is incredibly — almost frightfully — contagious, 95 percent of people in any given community must be immune to stop its transmission. But today, only 28 percent of US counties vaccinate enough of their kindergarteners to meet that threshold. Another record-setting year for measlesView LinkLower vaccination rates are, in large part, a result of political polarization, misinformation, and declining trust in health authorities and institutions. But there’s another, more ironic reason for measles’ resurgence: We did such a good job getting rid of it.As late as the 1950s, virtually every American child contracted measles before age 16. Roughly 48,000 patients were hospitalized each year with measles, which is among the most contagious infections known to humanity. But the development of a highly effective vaccine in the 1960s, and a series of aggressive public immunization campaigns in the decades after that, all but eradicated a once-common childhood illness. The federal government poured millions of dollars into vaccination programs, especially for low-income kids. Everyone from Michael Jackson to the Teenage Mutant Ninja Turtles championed measles immunization. All that campaigning worked. By the end of the 1990s, more than 90 percent of Americans had received the measles vaccine, and a similar percentage agreed that it was important to vaccinate children. As a result, most people born in the last 40 years have no firsthand experience of how dangerous measles is.“We’ve done too good of a job,” one doctor joked to Vox last December. In other words, ironically, America’s success at vaccinating against measles … has made Americans complacent about measles vaccination. One link for later➨ Go with your gut. The hashtag #GutTok has over 6 billion views on TikTok, and viral trends that promise to “hack your gut” through miracle products and cleanses are all the rage. But you don’t need an influencer-endorsed supplement for digestive wellness. Instead, actual experts recommend basic lifestyle steps like eating a high-fiber diet and reducing stress. Before you go… Did you know…that there are more endangered species in California than in any other US state? But more surprising is the runner-up: After California, Alabama has the most endangered species. Today’s trivia: Which Irish musician sings the song “Orinoco Flow”? (You can find this and other brain puzzles in Vox’s daily crossword. Look for the answer in tomorrow’s edition.)Yesterday’s trivia: On Friday we asked you for the four-letter university located in Athens. That would be Ohio. But there are more than two dozen Athenses in the US, and a rival Athens is also home to the University of Georgia.