WASHINGTON — The ongoing measles outbreak in the United States has reached a record for any year since the disease was eliminated in this country 14 years ago, with 288 cases of the potentially deadly infection reported in 18 states, the Centers for Disease Control and Prevention said Thursday.

The largest measles clusters are in Ohio (138 confirmed cases), California (60) and New York (26), according to the CDC. Almost all — 97 percent — have been brought into the country by travelers, mainly Americans, who contracted the infection abroad. About half of the infections were picked up in the Philippines, where a large measles outbreak has affected more than 32,000 people, causing 41 deaths between Jan. 1 and April 20, according to the CDC.

In this country, the biggest outbreak is centered in the Amish community in Ohio, where many residents are unvaccinated, the CDC reported.

“This is a wake-up call for travelers and parents to make sure vaccinations are up to date,” said Anne Schuchat, director of the CDC’s National Center for Immunization and Respiratory Diseases.

“Measles vaccine is very safe and effective, and measles can be serious,” she added. “It’s very infectious.”

Forty-three of those who have come down with measles in the United States required hospitalization, most often for pneumonia, Schuchat said. No deaths have been reported here.

Measles is a highly contagious respiratory disease that generally affects young children, causing fever, a runny nose, a cough and a distinctive rash all over the body. This year, however, more than half the people who have come down with it are 20 years old or older, according to CDC data.

About one in 10 children with measles also get ear infections, and one in 20 develop pneumonia. A person with measles is contagious for as long as four days before symptoms are apparent. Parents and even doctors who have not seen measles in years may not be aware of the early signs.

The largest number of confirmed cases of measles since the infection was eliminated in the United States in 2000 occurred in 2011, when 220 were recorded. The CDC has not seen this many cases so early in a year since 1994, when 764 people were infected by this time, Schuchat said.

In the past 20 years, a public health campaign, especially targeting lower-income families, has made measles outbreaks rare in the United States. But an estimated 20 million people are infected in Europe, Asia, Africa and elsewhere each year, and 122,000 die.

In the United States, the number of people who choose not to be immunized for religious, philosophical or personal reasons has begun to become a public health problem, Schuchat said. Others are unaware of, or are unable to get, vaccinations before they arrive in the United States. A small number of adults can lose their immunity over time and may need to be revaccinated.

Authorities are not sure how the disease entered the Amish community in Ohio, but Schuchat said they believe that people traveling to conduct faith-based work abroad are involved. According to the CDC, 40 importations of the infection were attributed to unvaccinated U.S. travelers returning from abroad.

Steven Nolt, a professor of history at Goshen College in Indiana who has written about the Amish, said members of some groups do travel to places such as Kenya, Ukraine and Central America to do mission or relief work. He said that many Amish do have themselves and their children vaccinated but that others refuse.

Some, Nolt said, have a “more traditional, conservative, old-fashioned way of life and set of sensibilities that views medicine as something that is used to heal or cure, rather than to prevent” disease. Others have a “theologically informed . . . sense that we should place our trust in God and not in vaccines.”

Schuchat urged anyone who is unsure whether his or her immunizations are up to date to get another dose of the vaccine, especially if traveling to places such as the Philippines or doing health-care work.

Although the vaccine generally is not given to children before the first birthday, infants as young as 6 months old who are being taken abroad can be inoculated with one dose, Schuchat said. The vaccine is generally administered in two doses a few years apart. Pregnant women and those whose immune systems are suppressed should not receive the vaccine, she said.

Those born before 1957 are likely to have had the measles and should be immune, she added. The vaccine became available in 1963.