A disease that American medicine once brought to the edge of elimination is finding its way back into communities, carried not by chance but by the slow erosion of the collective immunity that once held it at bay. Measles is circulating again in the United States, and health authorities are now asking certain adults — particularly those vaccinated during an earlier, less reliable era of immunization — to examine whether their protection has held. The resurgence reflects something larger than a single virus: it is a measure of how much trust in public health has frayed, and how quickly hard-won