Just as one large measles outbreak peters out in the United States, another outbreak of the virus has taken off along the border of Utah and Arizona.

The new outbreak began in August and has sickened more than 100 people, making it the second-largest cluster of cases in the country this year. A majority of the cases are in unvaccinated people.

It comes during an already bleak year for the nation’s public health: The number of measles cases hit a 34-year high this summer, largely driven by the so-called “Southwest outbreak,” which grew to more than 880 cases across Texas, New Mexico and Oklahoma. Several epidemiologists agreed that the current scale and spread of cases most closely resembles the large outbreaks of the early 1990s — before nationwide immunization campaigns and school vacci

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