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... With baby Arthur too young for the measles vaccine and a sibling due in June, the Otwells grew nervous when the threat of the highly contagious virus started factoring into their grocery run.

"We go to the Costco that was kind of a hotbed," said John Otwell, who knew about the state health department's warnings of public exposures at the store. "A lot of people just don't get it; they think it's just a cold. It's not."

By Arthur's 9-month checkup, the South Carolina outbreak had exploded into the nation's worst in more than 35 years, surpassing last year's in Texas. That meant that under state guidance, Arthur could get his first dose of the MMR vaccine " for measles, mumps and rubella " earlier than the usual 12 to 15 months old. Their new baby won't be able to get the shot until at least 6 months " a prospect that worries parents of infants wherever measles spreads.

Babies too young to be vaccinated are among the most vulnerable in a measles outbreak. The disease can wreak havoc on their fragile bodies, making them so sick they stop eating and drinking. They can develop pneumonia or brain swelling, and sometimes die.

Babies depend entirely on herd immunity -- at least 95% of a community must be vaccinated to prevent measles outbreaks.

But dropping vaccination rates have eroded protection in South Carolina and across the nation. In Spartanburg County, the outbreak's epicenter, less than 90% of students have gotten required vaccines. ...