Last year, starting in January, the United States experienced its largest documented measles outbreak in more than three decades, when an epidemic centered on West Texas sickened at least 762 people. Now a fast-moving outbreak in South Carolina seems poised to surpass it: Local officials have logged 700 infections, and the virus is still rapidly spreading. As public-health officials scramble to contain the virus, they're also trying to figure out whether these two outbreaks are connected'specifically, whether the version of the pathogen that kick-started the West Texas cases has been circulating within the nation's borders ever since. If the answer is yes, it will mean that measles has once again become a permanent resident of this country, after 26 years of only limited outbreaks imported from abroad. Given that the U.S. clocked more than 2,200 measles cases in 2025'more than it has had in a single year since 1991'the experts I spoke with already consider this the reality that...
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