At least 695 people have been infected with measles this year as of Wednesday, an alarming rate for an infectious disease that was declared eradicated in the U.S. in 2000.

The Centers for Disease Control and Prevention said that 22 states had been affected but that most cases were clustered in Washington state and New York.

"The longer these outbreaks continue, the greater the chance measles will again get a sustained foothold in the United States," the agency said in a release.

Measles, once a common childhood disease that infected 4 million a year, is characterized by a red skin rash.

Most people who caught the virus this year were not given the MMR vaccine to protect against measles, mumps, and rubella. Children are supposed to get it around the age of 1 and again before they enter kindergarten. The first vaccine has a 95% effectiveness rate, and the booster has a 99% protection rate.

CDC said that a leading factor contributing to the outbreak in New York was that people were spreading "inaccurate and misleading" information about the vaccine. People who are skeptical of vaccinations tend to raise a now-debunked study claiming that the MMR vaccine caused autism. President Trump spread that same message when he was running for office, but has not tweeted about the current outbreak.

Trump's health and human services secretary, Alex Azar, took a forceful tone, urging people to get vaccinated in a statement Wednesday.

[Read more: How the measles virus is infecting hundreds, after being eliminated in 2000]

"Vaccines are a safe, highly effective public health solution that can prevent this disease," he said. "The measles vaccines are among the most extensively studied medical products we have, and their safety has been firmly established over many years in some of the largest vaccine studies ever undertaken."

Measles is hard to contain because it’s not only highly contagious — spreading via infected surfaces, among people, and through the air — but also because people are contagious for four days before and after they get a rash. This means people start to have symptoms such as runny nose, cough, and red eyes without knowing they have measles because they haven’t broken out in the tell-tale rash.

People who get measles become immune for life, but the illness can be much more dangerous and even deadly for some people. Before the vaccine, 48,000 people were hospitalized in the U.S every year from the disease, and 500 died. Another 1,000 developed brain swelling known as encephalitis.

Schools require children to get vaccinated, but some states allow exceptions for religious or philosophical reasons, and some parents choose not to vaccinate their children because they fear vaccines are unsafe. In other instances, parents try to space out vaccines and fall behind. Adults also are vulnerable because the U.S. didn't start recommending two vaccines until 1989, meaning that many adults haven't had the booster shot not because they are actively opting out but because they were never urged to get the vaccine.

The last time a measles outbreak was close to the current level was in 2014, when 667 people were infected. In that instance, the outbreaks were clustered in Amish communities in Ohio where people had not been vaccinated. In this outbreak, clusters are occurring in New York state, New York City, and New Jersey, primarily among unvaccinated people in Orthodox Jewish communities.

[Related: Anti-vaxx organization sues New York over measles vaccine mandate]