Decades of progress in eliminating measles are slowly being erased as vaccination rates against the disease decline across the country, threatening communities with potentially deadly outbreaks of cases from the highly infectious virus, health officials say.

“If ever there was a disease that you really wanted to vaccinate everyone against, it would be measles,” said Dr. Anthony Fauci, director of the National Institute of Allergy and Infectious Diseases, part of the National Institutes of Health. “It’s got all the right ingredients: It’s highly transmissible, potentially really serious infection and has a vaccine that is incredibly effective.”

At least three outbreaks have been reported across the country: two in New York and one in Washington state. An outbreak in New Jersey was declared over in mid-January.

Last year marked the second-highest number of measles cases, 349, since the disease was declared to be eliminated in the U.S. in 2000. The worst year was 2014, when 667 cases were reported.

The increase was largely attributed to the number of families and communities that refused vaccination on religious and philosophical grounds.

Measles cases around the world have increased by 30 percent, and the World Health Organization warned last week that vaccine refusal is one of the top 10 global health threats.

“When you have a certain proportion of the population that does not want to get vaccinated, you get below the critical level of herd immunity,” Dr. Fauci said, “and then when somebody comes in from the outside who’s infected, then you get the outbreaks that we’re seeing. … It’s very, very unfortunate. It always bothers me a lot because measles is such an amazingly effective vaccine.”

Such is the case for current outbreaks in New York City and Rockland County, New York — with 58 and 118 diagnoses, respectively — which are largely in Orthodox Jewish communities. The outbreaks have been traced to an unvaccinated child who recently returned from a foreign country where measles is either rampant or endemic.

The virus is incredibly contagious, spread through the air by sneezing, coughing and even talking when spittle gets into the air. If droplets land on surfaces, they can remain infectious for several hours.

Symptoms don’t appear until about one to two weeks after infection, at which point a rash and raised red bumps appear on the face and spread over the body. Other symptoms include fever, cough, sore throat and inflamed eyes.

About 1 in 10 of those infected, especially children, will develop an ear infection. About 1 in 20 will develop pneumonia, and about 1 in 1,000 will die.

The measles vaccine — known as MMR for measles, mumps and rubella — has been available since the 1960s. Today, it is given to children in two doses before age 2. The vaccine’s effectiveness against measles is 93 percent to 97 percent.

Yet herd immunity — a population’s resistance to the spread of a communicable disease — is effective only if 95 percent of a population is inoculated.

The national rate for MMR vaccination is a little over 91 percent, but this varies greatly among localities.

Four states — Massachusetts, Virginia, North Dakota and Rhode Island — have vaccination rates for MMR above 95 percent.

Clark County, Washington, which is experiencing an outbreak, has some of the lowest vaccination rates in the state, at about 77 percent.

“One measles case is like Defcon 1,” said Dr. Dawn Nolt, an infectious disease specialist with Doernbecher Children’s Hospital in Portland, Oregon, which sits across the river from Clark County.

In Oregon and Washington, Dr. Nolt said, health officials are fielding calls from people who fear they might have been exposed. Officials have listed dozens of schools, health care facilities and public places where people might have been exposed, including a Portland Trailblazers basketball game in January.

“Overall, immunization rates have been decreasing, unfortunately, due to the rise of the ability of parents to have nonmedical exemptions against vaccinations,” she said.

Although all states allow a medical exemption for vaccinations, 48 states allow parents to refuse vaccinations for their children on religious grounds and 20 states allow refusal on philosophical grounds, according to a 2016 study.

“We have made several efforts, the physicians here in the state, as well as in Washington, annual efforts to our legislature to try to make vaccine exemptions be only for medical reasons, and it has not been successful,” said Dr. Nolt, who is also an associate professor of pediatrics.

According to an October report from the Centers for Disease Control and Prevention, the percentage of children who received no vaccinations by age 2 increased from 0.9 percent in 2011 to 1.3 percent in 2015. The percentages were highest among children who were covered under Medicaid and lived in rural communities or far away from cities.

In 2014, one of the largest measles outbreaks — almost 400 — occurred in an Amish community in Ohio. In California, an outbreak at Disneyland revealed that 78 percent of the 125 cases occurred in children who were intentionally unvaccinated.

In 2017, about 75 cases of measles were reported in a Somali-American community in Minnesota whose children hadn’t been vaccinated over fears that they would develop autism, a myth that health officials have debunked.

Dr. Nolt said families she encounters who are hesitant about vaccinations typically vacillate between misinformation and safety fears.

“A lot of it is concerns that they’ve never seen these diseases and, hence, why do we need to prevent them and the vaccines probably won’t work anyway because they’ve never seen these diseases,” she said.

“Many of them just want to hear more information, and once you talk to them about safety and science and really that these are evidence-based recommendations, they will go for it,” Dr. Nolt said.