Reported measles cases in South Carolina surged by almost 30% in the last few days, state health officials said Friday.

The South Carolina health department reported 124 new cases since Tuesday, bringing the state’s total to 558 in a wave of infections centered around an outbreak in Spartanburg County.

Since the holidays, South Carolina’s measles outbreak has exploded into the worst in the U.S. Measles also have been reported this year in Arizona, Florida, Georgia, North Carolina, Ohio, Oregon, Utah and Virginia, according to the Centers for Disease Control and Prevention.

Last year was the nation’s worst for the spread of measles since 1991, according to the CDC. The U.S. confirmed 2,144 cases across 44 states. Three people died, all of them unvaccinated.

Measles is caused by a highly contagious virus that is airborne and spreads easily when an infected person breathes, sneezes or coughs. It is preventable through vaccines and has been considered eliminated from the U.S. since 2000, though the country is at risk of losing that achievement.

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