The number of confirmed measles cases across the U.S. has reached 839, and the country is experiencing its worst outbreak of the disease since 1994.

The Centers for Disease Control and Prevention reported Monday that the tally for the year had jumped by 75 within a single week as of May 10, and that the outbreak had affected 23 states.

The crisis is hitting New York the hardest; 66 of the new cases have been reported there, according to CNBC. Forty-one of those were reported in New York City, which in April announced mandatory vaccinations in Brooklyn and declared a public health emergency within parts of the borough.

Arizona, California, Colorado, Connecticut, Florida, Georgia, Illinois, Indiana, Iowa, Kentucky, Maryland, Massachusetts, Michigan, Missouri, Nevada, New Hampshire, New Jersey, Oregon, Pennsylvania, Texas, Tennessee and Washington are also reporting cases.

The CDC has linked outbreaks ― which are defined as three or more cases ― in seven states to “travelers who brought measles back from other countries such as Israel, Ukraine, and the Philippines, where large measles outbreaks are occurring.”

Measles was declared eliminated in 2000 and is preventable with immunizations, the agency notes, but it can be spread within communities containing groups of unvaccinated people.

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