(CNN) — The number of measles cases in the United States made its biggest jump of the year, with 90 new cases reported in just one week, according to numbers released Monday by the US Centers for Disease Control and Prevention.

With 555 total cases, 2019 now has the second-highest number of measles cases in the United States in 25 years — and the year isn’t even half over.

READ MORE: Gov. Newsom Makes California’s Voting By Mail System Permanent

“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. “This is not going to stop on its own.”

Measles isn’t just rising in the United States. The World Health Organization reported Monday that there were more than 110,000 measles cases worldwide in the first three months of 2019, an increase of nearly 300% from the same period last year.

The data is provisional, and the actual number of measles cases is probably higher, as WHO estimates that fewer than 1 in 10 cases globally is reported to the agency.

Ukraine had the highest number of cases in the past 12 months, with more than 72,000, followed by Madagascar and India with more than 69,000 and 60,000 cases, respectively. WHO warned that there are delays in reporting and that this data may be incomplete.

A ‘fast-moving, life-threatening disease’

“By the time you finish reading this, we estimate that at least 40 people — most of them children — will be infected by this fast-moving, life-threatening disease,” Henrietta Fore, executive director of UNICEF, and Dr. Tedros Adhanom Ghebreyesus, WHO director-general, wrote in an opinion piece Monday on CNN.

About 1 out of every 1,000 children who gets measles will develop encephalitis or swelling of the brain, according to the CDC. This can lead to convulsions and leave a child deaf or with an intellectual disability.

Additionally, 1 or 2 out of 1,000 children who get measles will die from it.

No fatalities have been reported in the United States from measles this year or last year, but 35 people in European Union countries died of the disease in 2018, according to the European Centre for Disease Prevention and Control.

Before the first measles vaccine became available in 1963, the disease killed hundreds of people and hospitalized 48,000 each year, according to the CDC.

After the vaccine, cases plummeted, with 963 cases in 1994. In 2000, the disease was declared eliminated in the United States. Despite the ongoing outbreaks in communities across the United States, measles is still considered eliminated, which means it is not being continuously transmitted in this country. Measles would no longer be considered eliminated once it was continuously transmitted for longer than 12 months.

The CDC recommends two doses of the measles mumps and rubella, or MMR, vaccine for children. The first should be given at 12 to 15 months and the second when the child is 4 to 6 years old. The first dose gives 93% protection against measles and the second dose gives 97% protection.

READ MORE: Sheriff: Car-To-Car Shooting Near Large Party In Yuba City Leaves 1 Hurt

The anti-vaxer movement

Experts point to one reason for this year’s large outbreak: the power of the anti-vaccination movement.

“It’s just terribly sad that children in the US are having to suffer measles. This should not happen,” said Dr. William Schaffner, medical director of the National Foundation for Infectious Diseases and an adviser to the CDC on vaccines. “We’d previously eliminated this disease not just in the US but in the entire Western Hemisphere, and it appears that now we’ve profoundly and sadly turned back the clock.”

The 20 states reporting measles this year are Arizona, California, Colorado, Connecticut, Florida, Georgia, Illinois, Indiana, Kentucky, Maryland, Massachusetts, Michigan, Missouri, Nevada, New Hampshire, New Jersey, New York, Oregon, Texas and Washington.

Most of the cases have been in New York, site of an outbreak among ultra-Orthodox Jews that started in the fall.

Messonier says the New York outbreak has been particularly difficult to control. “Most measles outbreaks in the US stop sooner than this,” she said.

Health officials there announced last week that in the neighborhoods affected by the outbreak, anyone who has not had been vaccinated against measles or cannot show evidence of immunity could face a $1,000 fine.

On Monday, the New York City health department announced that one child-care program was closed “for repeatedly failing to provide access to medical and attendance records in violation of an Order of the Health Commissioner in response to the current measles outbreak.” Schools and child-care programs are required to maintain records on-site, and unvaccinated students and staff are prohibited from attending.

In addition, the department said, “23 yeshivas and day care programs have received Notices of Violations (NOVs) for not following the school exclusion order.”

Health officials in Rockland County, New York, tried to bar unvaccinated children from public places, but a judge prohibited the county from enforcing that rule.

Messonier said it’s a matter of “correcting myths” about vaccination. Health authorities have worked with rabbis to explain that vaccination is safe, but that still hasn’t turned the outbreak around.

“You have to just approach people where they are and answer their questions,” Messonier said. “It’s about the slow work of developing trust.”

MORE NEWS: R. Kelly Found Guilty Of All Counts In Sex Trafficking, Racketeering Trial

™ & © 2019 Cable News Network, Inc., a Time Warner Company. All rights reserved.