According to the Centers for Disease Control and Prevention, the US has been experiencing its second-worst measles outbreak since the highly contagious and risky disease was declared eliminated in 2000.
But rich countries with seemingly high vaccination rates are seeing cases spike too.
It is one of the most contagious viruses around, however, nothing about measles has changed.
"The disease has spread fast among clusters of unvaccinated people", it added.
The U.S. outbreak is part of a worldwide rise. Of those, 229 were reported this year, accounting for more than one-third of the 555 cases that have been reported nationwide in 2019, as of April 11, according to figures updated Monday by the CDC.
A growing and vocal fringe of parents in the United States oppose measles vaccines in the belief, contrary to scientific evidence, that ingredients in the vaccines can cause autism or other disorders.
NY health officials said they were working with orthodox Jewish leaders to combat anti-vaccination campaigns.
States that have reported cases to CDC include NY, where there have been outbreaks in Brooklyn and Rockland County, New Jersey, Connecticut and MA.
New York City has confirmed 329 cases of measles since October, including 44 since last week's emergency order, according to local health officials, the large majority among children of the Orthodox Jewish communities in Brooklyn.
"By the time you finish reading this, we estimate that at least 40 people - majority children - will be infected by this fast-moving, life-threatening disease", Henrietta Fore, executive director of UNICEF and Dr. Tedros Adhanom Ghebreyesus, director general of the World Health Organization, wrote in an opinion piece Monday on CNN.
"I'm obviously very concerned about the size and also acceleration of the current outbreak", said Dr. Nancy Messonier, director of the CDC's National Center for Immunization and Respiratory Diseases.
Of the 329 confirmed NY measles cases, 284 are children and 45 are adults.
NY health officials said the Brooklyn outbreak was caused by an unvaccinated child returning from a trip to Israel, which is also grappling with an outbreak.
Several people had to be hospitalized, but no fatalities had been reported as of last week.
A state judge overturned the order, but Rockland is appealing that decision.