Measles, a highly contagious but preventable disease, is resurging in pockets of the United States, a warning of the dangers of the strengthening anti-vaccine movement.

The Centers for Disease Control and Prevention has recorded more cases this year than the 58 tallied in all of 2023, although the agency is not expected to release exact numbers until Friday. On Monday, the agency advised health care providers to ensure that unvaccinated patients, especially those traveling internationally, stay updated on their immunizations.

The number of cases is likely to keep rising because of a sharp spike in measles worldwide, along with spring travel to some regions with outbreaks, including Britain, said Dr. Manisha Patel, chief medical officer at the C.D.C.’s respiratory disease division.

Nearly all the cases in the United States so far are related to unvaccinated travelers. “We’re not going to see widespread measles cases going throughout the country,” Dr. Patel said. “But we do expect additional cases and outbreaks to happen.”

Measles is among the most contagious of diseases; each infected person can spread the virus to as many as 18 others. The virus is airborne and can stay aloft up to two hours after an infected person has left the room, spreading rapidly through homes, schools and child care facilities.

In Chicago, one case of measles at a migrant shelter has grown to 13, prompting the C.D.C. to send a team to help contain the outbreak. (Two additional cases in the city appear to be unrelated.)

In Florida, seven students at an elementary school contracted measles even as the state’s surgeon general, Dr. Joseph Ladapo, left it to parents to decide whether unvaccinated children should attend school.

In southwest Washington, officials identified measles in six unvaccinated adult members of a family living in two counties. And in Arizona, an international traveler infected with measles dined at a restaurant and transmitted the virus to at least two others.

Measles was eliminated in the United States in 2000, and American children generally must be immunized to attend school. Yet sporadic cases lead to larger outbreaks every few years. But now a drop in vaccination rates, exacerbated by the coronavirus pandemic, has experts worried about a resurgence.

When vaccinations lag, “the first disease to appear is measles, because it’s highly infectious,” said Dr. Saad Omer, dean of the O’Donnell School of Public Health at U.T. Southwestern in Dallas.

Nine of 10 unvaccinated people in close contact with a measles patient will become infected, according to the C.D.C.

Measles is far less deadly in countries with high immunization rates and good medical care. Fewer than three of every 1,000 American children with measles will die as a result of severe complications like pneumonia or encephalitis, the swelling of the brain.

Still, about one in five people with measles may end up in a hospital.

Because widespread measles outbreaks have been rare, most Americans, including doctors, may not recognize the vibrant red rash that accompanies respiratory symptoms in a measles infection. They may have forgotten the impact of the disease on individuals and communities.

“Most of our local health department folks have never seen a measles outbreak,” said Dr. Christine Hahn, state epidemiologist of Idaho, which contained a small cluster of cases last year.

“It’s going to be a big challenge to us to respond if and when we get our next outbreak,” she said.

Before the first measles vaccine was introduced in the 1960s, the disease killed an estimated 2.6 million people worldwide each year. But its full impact may have been much greater.

Measles cripples the immune system, allowing other pathogens easier entry into the body. A 2015 study estimated that measles may have accounted for as many as half of all infectious disease deaths in children.

For about a month after the acute illness, measles can stun the body’s first response to other bacteria and viruses, said Dr. Michael Mina, chief science officer of the digital health company eMed and formerly an epidemiologist at the Harvard T.H. Chan School of Public Health.

That leaves patients “massively susceptible to bacterial pneumonias and other things,” said Dr. Mina, who was the lead author on the 2015 study.

“It’s very risky for people in those first few weeks post measles,” he added.

The virus also induces a sort of immune-system amnesia. Normally the body “remembers” the bacteria and viruses it has fought before. Dr. Mina and his colleagues showed in 2019 that people who have measles lose between 11 and 73 percent of their hard-won immune repertoire, a loss that can last for years.

That does not mean the body no longer recognizes those pathogens at all, but it does shrink the arsenal of weapons available to fight them.

“People should be aware that if they’re choosing not to vaccinate, that’s the position they’re putting themselves and their family in,” Dr. Mina said.

The C.D.C. recommends receiving the first dose of the measles vaccine after 12 months of age, and a second between ages 4 and 6. Even a single dose of the vaccine is 93 percent effective. Measles vaccination averted 56 million deaths between 2000 and 2021, according to the World Health Organization.

Vaccination rates in the United States have shown a distinct, if small, dip to 93 percent in the 2022-23 school year from 95 percent in 2019-20 — the level required to protect everyone in the community. Rates of vaccination exemptions increased in 40 states and the District of Columbia.

In a survey last year, just over half of Republicans said that public schools should require measles vaccinations, compared with about 80 percent before the pandemic. (Support for vaccines among Democrats held steady.)

While national or state-level vaccination rates may be high, there may be pockets of low immunization that provide tinder for the measles virus, Dr. Omer said.

If there are enough unvaccinated cases to sustain an outbreak, even those who are vaccinated but whose immunity may have waned are vulnerable, he said.

In Idaho, 12 percent of kindergarten-age children do not have a record of vaccination. Some of the gap results from parents unable or unwilling to share records with the schools, and not because their children are not immunized, Dr. Hahn said.

Still, online schools, which proliferated through the pandemic and remain popular in the state, have some of the highest rates of vaccine exemptions, she said.

In September, a young Idaho man brought measles back after international travel and became ill enough to be hospitalized. Along the way, he exposed fellow passengers on two flights, dozens of health care workers and patients, and nine unvaccinated family members. All nine developed measles.

Idaho got “very lucky” with the outbreak because the family lived in a remote area, Dr. Hahn said. But there are most likely many other areas in the state where an outbreak would be difficult to contain.

“We’ve got plenty of tinder, if you will,” she added.

Some large outbreaks in recent years exploded among huge clusters of unvaccinated people, including the Amish in Ohio and the Orthodox Jewish community in New York City.

In September 2018, one unvaccinated child returned to New York City from Israel, ferrying measles virus picked up during an outbreak in that country.

Even though the city maintains high vaccination rates, that single case set off an outbreak that raged for nearly 10 months, the largest in the country in decades. The city declared a public health emergency for the first time in more than 100 years.

“We had more than 100 chains of transmission,” said Dr. Oxiris Barbot, the city’s health commissioner at the time, and now the president and chief executive of United Hospital Fund.

“Keeping all of that straight was a challenge,” she recalled. “And to have to investigate over 20,000 exposures like that, that was huge.”

Working with community leaders, city officials hurriedly administered about 200,000 doses of vaccine. More than 550 city staff members were involved in the response, and the final cost to the city’s health department topped $8 million.

The C.D.C. is working with state and local health departments to identify pockets of low vaccination and prepare them for outbreaks, Dr. Patel said. The agency is also training health care providers to recognize measles symptoms, particularly in patients with a history of international travel.

Measles is a slippery adversary, but public health is intimately familiar with the tools needed to contain it: screening, tracing contacts and vaccinating the susceptible.

“We’re not helpless bystanders,” Dr. Omer said. “The focus needs to be on meat-and-potatoes public health.”